Can someone sue me if they copyright a short word phrase for a tshirt, even though I used it first and didn't copyright it?

Your problem arises under trademark law, not copyright law. Ordinarily, short phrases are not protected by copyright law (although there are some exceptions for famous song titles and song lyrics). But a short phrase can become a trademark if it has been used in commerce to identify the source of certain goods or services. If you began use a specific phrase in commerce to identify your T-shirts (and this may include printing the phrase on the T-shirts---think Abercrombie & Fitch), then you can gain common law rights to use that trademark even without registration. If you are the first company to use the trademark in commerce, you are the senior user. Even if someone else swoops in and registers the trademark for use in identifying its t-shirts, as the senior user you can block this junior user from infringing your common law rights---this is because the junior user has wrongly created consumer confusion. You can also apply for cancellation of the registration obtained from the USPTO by this junior user and instead, apply for registration of your trademark. Generally speaking, the senior user wins these battles. Thus, in this case, you can probably win this battle with the junior user----but it will be expensive. Your next move is to retain trademark litigation counsel to develop a strategy for protecting your rights. Unfortunately, this will be expensive----you need to be realistic about the costs here---it takes tens of thousands of dollars to fight trademark battles like this---in fact it is not unusual for legal fees in trademark cases to exceed several million dollars. Thus, this is one of those matters that you should try to settle----the role of IP counsel when dealing with small clients with limited budgets is to try to facilitate cost effective settlement. Good luck.

Generally, short phrases aren't considered to have sufficient" expression" to be protectable by copyright.