Question about having to recertify for intermittent FMLA after only 6 months? My supervisor and HR say my absences are "suspicious". Can they deny my FMLA request this time?

I work in a hospital as a radiology tech in Montana. Like my title said, I applied for, and was granted intermittent FMLA for chronic migraines in October. I've used just barely have of the leave (and it's been about six months). I got called into HR this morning and told that my absences are a "suspicious pattern" because they always seem to correspond to one of my days off, and so they don't believe that my leave is medically necessary anymore. And it just looks like I'm giving myself long weekends. Well, I work 4 10 hour shifts, Monday, Tuesday, Thursday and Friday. So pretty much any day I call off will correspond to one of my days off!

So they said they are making me re-certify for the FMLA. And they are also considering moving me to a different position, one that is easier to backfill if I have to call off sick (I really hope that doesn't happen, but I believe they are legally allowed to do that).

Are they allowed to make me re-certify for the FMLA like this though? 6 months in? Could they possibly deny it once the doctor turns the new paperwork in? Then what recourse would I have? What are my rights here?

Thanks for your time. I'm really scared now that they're trying to fire me, but can't, and so are trying to find some way to back me into a corner.