Wouldnt my first set of approved FMLA paperwork cover this and I feel like they are harassing me.

I have cluster migraines that put me down for a day or two each time I have them. Last month I had a several over the course of a few days and had to take 4 days in a row off. My previous fmla paperwork said I could have 1-2 flare ups a month that would last 1-2 days each. I took 4 days off and that would seem to be covered I had thought but they made me get recertified because they told me that would have been just one flair up and I took 4 days off. They have now told me the paperwork is not correct on my new FMLA and I am now on the 4th time taking it back to the Doctor to fill out, although the new fmla paperwork has more information then my previous approved one. I have been taken into the office with the HR manager and specialist and quized on what is going on why cant I work and what medicine i am on. During this discussion I informed them what was going on and everything they asked for but now HR is telling me that I need to have a certification from my GP, the neurologist that I am going to see and since i am a Gulf war veteran with some similarities to Gulf War Syndrome and I am going back to the VA for help they then informed me I would have to have them certifiy me also. Is this legal for them to do? What can I do to protect myself?