Your situation sounds very much like mine. You probably don't have the same thing but it would be worth asking your doctor about. I can't sit on my right butt cheek and it's quickly spreading into my left side. It comes up from the bottom of both butt cheeks. I was told that it's part of the pudendal (sp?) nerve. Is that what you're experiencing? Have you ever had a pudendal nerve block? My diagnosis is Tarlov cysts (perinerual cysts). They come out of the spinal cord and are innervated so they cannot be removed. There is a surgery available for them though. They drain the CSF out of them and fill them with another substance. There are only a few doctors in the U.S. that specialize in these.
Tarlov cysts are considered an incidental finding on MRIs, CTs, etc. The radiologist may or may not even document them. It has taken me 16 years to find a doctor that questioned what was seen on my MRI. The cysts were present back in 2003 after my accident. I've found that most doctors won't even acknowledge them and say they don't cause any issues. When they grow bigger they DO cause issues. Most times they are in the sacral area and push on the nerves at the end of the spinal cord and cause cauda equina syndrome.Do you have any of the cauda equina symptoms? It would be a really good idea to request all of your images on discs and the reports that go with them. From now on when you go in for any kind of imaging ask for a disk and that a copy of the report be mailed to you. They will give you a disc before you leave. Read through all of your reports and ask about the things you aren't familiar with. Also, I've found that talking to the tech. that asks you all the questions before your scan helps. Tell them you want all incidental findings documented because they haven't been able to pinpoint exactly what is causing your issue. That's what I had to do anyway.
I hope you find out what the problem is. I absolutely understand the agony that comes with not being able to sit! Best of luck to you:)