My Employer in Violation of FMLA Laws?

I have a chronic condition in which I was approved for intermittent FMLA leave for time off for doctor appointments, physical therapy and time off in the condition is exasperated. My physician wrote the FMLA certification as a "chronic condition with intermittent/indeterminant frequency.

For the most part I am able to work regularly if my pain is kept in check, but there are times in which the pain is so unbearable I can barely walk let alone work. I never IMO abused the FMLA, but recently had a need to take 1 day off because of the pain. While waiting for the doctor, my boss called and said they will need a doctor's note as I have been using FMLA leave a lot lately. Not true, it was for appointments, all of which were submitted in advance and for a few hours to see my doctor about the increase in the pain. Nonetheless, I saw the doctor, was referred to my pain management doctor and gave my employer a note which stated I was to be off for a few days, but I returned earlier than the note specified.

I believe this is in direct violation of my need for intermittent FMLA when in pain. If the certification states I am to be on FMLA for a duration of at least 6 months, can they request a doctor note each time that I do call out due to the pain during this time?