<3, I also take my own BP reading at home and I'm on BP medication. I don't have fibro, however. Here is what the NIH says about
taking blood pressure readings. There's a paragraph down the page on taking it on the forearm but the entire article is interesting to read:
www.ncbi.nlm.nih.gov/pmc/articles/PMC2911816/Because mine is usually higher in the doctor's office, I take mine randomly at home and it's always lower - of course! Anyway, the article does emphasize the importance of feet on the floor, back supported, arm supported and at heart level. Well, guess what, the nurse takes mine while I'm sitting on that darn table with my feet dangling and no back support.
If I were you, I'd keep a record of your readings and make a note of the day and time. Maybe you need to adjust your cuff to be a little looser? And absolutely, tell your doctor what you wrote here, about
the pain in the upper arm. Best Wishes.