Rheumatoid Arthritis Support Group
Rheumatoid arthritis is a chronic, inflammatory, multisystem, autoimmune disorder. It is a disabling and painful condition which can lead to substantial loss of mobility due to pain and joint destruction. The disease is also systemic in that it often also affects many extra-articular tissues throughout the body including the skin, blood vessels, heart, lungs, and...

I can't say my butt ever hurt. That's a new one.
California Lynn/Drea
if you actually mean your butt cheeks you got me. ask your SO or spouse to investigate (sorry could not resist).
unless you broke your what is it called coxus once upon a time and it is just flaring now? in general sitting sucks for my ra, period. and it did at work also. i have a lot of these memory foam pillows covered in different colors so they don't look like I am stocking up on foam...all around the house. i have these from amazon take a look - they help, like a lot.
depot
https://www.amazon.com/MochoHome-Round-Memory-Cushion-Velvet/dp/B011554KWW/ref=sr_1_1?ie=UTF8&qid=1476802835&sr=8-1&keywords=memory+foam+cushions+round
But no! I went to several doctors and a physiotherapist and I pulled a ham string. I have exercises and stretches, and it is a lot less worse than it was, but still hurts if I sit too long.
I would see your family doctor and go to a physiotherapist. I guess I need to do that again too. I read about athletes who pull ham strings and they recover. So why not me? Obviously my pathetic attempts at exercising are not working.
My mom also has had this for 10 years. I didn't get it at all till it happened to me. She was very active though, at least a reason for it. I have no idea what caused mine as I was inactive at the time. Sigh!
We have to remember that not everything is RA!