There is no law that says you cannot be termed while under a doctor's care, no.
However, depending on the exact circumstances, it may or may not have been illegal to term you. Please answer the following questions:
1.) How long have you worked for this employer?
2.) How many employees does the employer have within 75 miles of your location?
3.) In the 12 months immediately preceding your leave, had you worked 1,250hours for this employer?
4.) Did your employer know, in at least general terms, the reason for your absence?
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.