I was fired while undergoing medical treatment: Is it legal?

I was fired while undergoing medical treatment: Is it legal?:
Dear Evil HR Lady,

Barely four months shy of my five year anniversary, I was terminated. The reason listed on my paperwork was “performance.” Although I had been working on a formal performance improvement plan, the termination was still somewhat unexpected due (mainly) to the fact that I was (and would still be if I had any insurance) undergoing treatment for a medical disability. My doctor had sent several letters to my manager regarding the difficulty I was experiencing with my medications, and my FMLA paperwork had been submitted and approved several months prior. Three months before my termination, I had, both verbally and via email, notified my supervisor, manager, and HR representative of my diagnosis and had requested (a) transfer to a different position if they felt that I could no longer effectively perform my job duties. The EEOC is currently investigating my case.

My concern is this: How do I explain the situation to a potential employer? My attempts to secure new employment have all failed, thus far, and I can’t help but think that this whole situation has played a major role in my continued unemployment. Reason for leaving last employer is something every interviewer has asked, and I just don’t know how to answer. “I got fired for poor performance, but it’s okay because I’ve filed a complaint with the EEOC about it” doesn’t strike me as the ideal response.
To read the answer click here: I was fired while undergoing medical treatment. Is it legal?

DIGITAL JUICE