I was on FMLA and it expired while I was under the doctors care. My job has terminated me and I was wondering can I sue them?

FMLA is the only protection you have for termination due to absences caused by a health reason.  In rare, circumstances you might be able to argue for protection under the ADA if you have a disability or another discrimination statute if you believe others employees were allow more time off than you.  Again, those are rare claims and usually the employer is within its legal rights to fire you once FMLA runs out.