Can my employer ask for a doctors note confirming my disability when I have not asked for an accommodation?

My employer cut my work hours after I missed a considerable amount of time due to a car accident (with drs. notes) I also just found out this past year that I have lupus. My employer said she would review it after 90 days. After the 90 days and my attendance had improved I asked for my hours back and she denied my request until I bring her a doctors note confirming my diagnosis of lupus. Other people at my office have health problems and have never been asked for a doctors note confirming their illness or disability except me. Is this discrimination?