Can my Employer call for an Attendance Investigation after each Intermittent FMLA day I take?

Hello and thank you in advance for your help. I have been with the same company for over 21 years now. My attendance has never been exemplary. However, I am a good employee. My performance evals are always positive. I have been suffering from a back injury for over 10 years now and Fibromyaglia pain for over 8 years now. I have Intermittent FMLA for when my pain is so bad I can't work. My Employer however has been scheduling "Attendance Investigations" every time I use an FMLA day or two.

I am doing everything they ask. Getting all the Doctor's Notes they ask for. Sometimes they are late, because my Dr falls behind. My Manager recently called for an investigation because I had some outstanding Dr Notes. I went to the Dr and paid my copay just to sit there and have him generate a Dr's note for every FMLA day I took that the employer didn't have and was asking for. I am in extreme pain. There is not a moment that goes by that I am not conscious of my pain. Most days my pain level from 1-10 is between 7 & 8. On days I can't make it in or I am late for work, my pain level is a 9. If it hits 10, I have to go to the ER because my Blood Pressure skyrockets. I am so stressed out because of the Investigations, I worry about losing my job on a daily basis. I am literally losing sleep over this.

So my question is, can my employer keep harassing me with "Attendance Investigations"? My FMLA Notes is for 4 days a month. Sometimes I use more than that, because I can't determine an exact amount of days a month I need due to the nature of the Fibromyalgia Disease. I don't even dare call in sick for my sick child either or myself.

After turning in all those Dr's notes my employer was asking for, another "Attendance Investigation" has been scheduled!