Problems with illness/employer

My employer has a three doctor excuse per calander year limit. I have used all 3 doctor excuses, 2 of those were for the same medical condition and additional 2 more for a total of 4 for the same condition. The employer takes anything over the 3 doctor excuses, even if they are medically documented and written excuses from the doctor, the employer still considers those days abscent and hold them against my attendance. Basically my doctor excuses are held against me as days of abscense. What can I do about this since my employer has not offered me a family medical leave act. But has offered FMLA for the medical testing of my illness. The employer keeps informing me that if I turn in any more doctor excuses, that I could be terminated. What are my rights? Thanks for any help you can give me.
Jim