Can a lawyer represent an opposing party if they represented me before? Is this a conflict?

No, probably not. Most likely there is a conflict of interest, but without knowing the nature of the new civil lawsuit, no one can really conclude one way or the other. An attorney may be disqualified if the former client can show that they had a “direct professional relationship…in which the attorney personally provided legal advice and services on a legal issue that is [substantially] related to the legal issue in the present representation.” (City and County of San Francisco v. Cobra Solutions, Inc. (2006) 38 Cal.4th 839, 847 (citing Jessen v. Hartford Casualty Ins. Co. (2003) 111 Cal.App.4th 698, 709). (If “the attorney had a direct [i.e. personal] professional relationship with the former client… the former client need not prove that the attorney possesses actual confidential information.”); see also Id. at 846 (“[A]n attorney may not switch sides during pending litigation representing first one side and then the other.”). In determining whether an attorney should be disqualified by reason of a conflict of interest, two rules are potentially implicated. The first is the per se rule of disqualification which generally prevents an attorney from undertaking a representation which is adverse to a current client. The attorney's duty of undivided loyalty prevents such simultaneous representation, even on matters which are wholly unrelated, without the informed written consent of both clients. The second rule governs successive representation of clients with adverse interests. An attorney is forbidden from undertaking a representation adverse to a former client if there is a “substantial relationship” between the current and former matters. This prohibition stems from the attorney’s duty to maintain client confidences. (Morrison Knudsen Corp. v. Hancock, Rothert & Bunshoft (1999) 69 Cal.App.4th 223, 230.) Pursuant to Rule of Professional Conduct 3-310(D), an attorney may not accept employment adverse to a client or former client where, by reason of the representation, the attorney has obtained confidential information. The purpose of the rule is to protect the confidential relationship, which continues even after the formal relationship ends. The fiduciary nature of that relationship requires the application of strict standards. In determining whether there is a substantial relationship between the former representation and the current representation, the court looks to “the practical consequences of the attorney's representation of the former client” and considers “whether confidential information material to the current dispute would normally have been imparted to the attorney by virtue of the nature of the former representation.” (H.F. Ahmanson & Co. v. Salomon Brothers, Inc. (1991) 229 Cal.App.3d 1445, 1454.) In doing so, the court focuses “on the similarities between the two factual situations, the legal questions posed, and the nature and extent of the attorney's involvement with the cases,” including “the time spent by the attorney on the earlier cases, the type of work performed, and the attorney’s possible exposure to formulation of policy or strategy.” (Id. at p. 1455.) Where there is a substantial relationship between the current and former representation, a conclusive presumption is triggered that the attorney possesses confidences that could prejudice the former client in the present matter. The substantial relationship test is broad and not limited to the strict facts, claims and issues involved in a particular action. The legal theories and issues that an attorney discusses with a former client may be different than those involved in the subsequent lawsuit against that client. Where an attorney acquires knowledge about the former client’s “attitudes,” practices, business customs, “litigation philosophy,” strengths, weaknesses or strategy, disqualification may be required for that reason alone. (Knight v. Ferguson (2007) 149 Cal.App.4th 1207, 1213-1215; citing Jessen v. Hartford Cas. Ins. Co. (2003)

This is a very complicated issue and you should consult an attorney. While all of the advice given previously is excellent, what concerns me is the relationship between you and the "attorney sub". What was the nature of the relationship? Was he or she making a special appearance on behalf of the other attorney? Or was the attorney a member of the law firm that you hired? All of these issues my greatly impact whether the attorney can be forced to sub out of the civil matter because of prior representation/attorney client privilege issues. This is my mind is different than a "conflict of interest" as mentioned in your question. A conflict of interest is one where the lawyer's interest and your's are directly opposed, such as his representing the victim for civil damages related to your criminal event or the attorney is suing you related to a business dealing (other than attorney's fees). Generally, there may be no conflict of interest or attorney-client privilege issue, if the lawsuit is not related to the criminal matter, no confidential information was exchanged, and there is sufficient time between the representation in the criminal matter and the new civil lawsuit.

The fact that he represented you before could create a conflict, but not always one sufficient enough to disqualify him from representing the other party. The original case was criminal, and this one appears to be civil. There may be enough difference in the two that it would be okay. Make sure that whoever you get to represent you in the case is aware of the issue, and they can determine what to do.

California’s Rules of Professional Conduct, Rule 3-310 (E) provides that an attorney “shall not, without the informed written consent of the client or former client, accept employment adverse to the client or former client where, by reason of the representation of the client or former client, the member has obtained confidential information material to the employment.” A former client, therefore, may seek to disqualify a former attorney from representing an adverse party where the former attorney possesses confidential information adverse to the former client. (H. F. Ahmanson & Co. v. Salomon Brothers, Inc. (1991) 229 Cal.App.3d 1445, 1452 [280 Cal.Rptr. 614].) In considering an attorney’s possession of confidential information, courts apply a “substantial relationship” test. (Global Van Lines, Inc. v. Superior Court (1983) 144 Cal.App.3d 483, 487-488 [192 Cal.Rptr. 609].) The former client need not show anything more than that the matters in the current lawsuit are “substantially related” to the matters in the case in which the attorney previously represented him. The Court will assume that during the course of the former representation confidences were disclosed to the attorney bearing on the subject matter of the representation, and will not inquire into their nature and extent. (See, River West, Inc. v. Nickel (1987) 188 Cal.App.3d 1297, 1302 [234 Cal.Rptr. 33].) You did not indicate the nature of the lawsuit in which your former attorney is now representing the other party. Without knowing that, one cannot determine if the previous criminal matter would be "substantially related" to the current lawsuit. Also, is the former attorney the one suing you, or does he simply represent a party who is suing you? If the attorney himself is suing you (e.g., recovery of attorney's fees), the Rules of Professional Conduct section indicated above may not even apply.