What Causes A Painful Lump Near The Anus?

I have a lump inside my buttocks next to my anus and it has become painful like a strained muscle. I haven t had an ultrasound, but did see a doctor. I spend a lot of time sitting between work and school, and had pain in my lower back, so my doctor thought the pain (didn t have a lump at that time) could be tight muscles running from sciatic nerve in my back down to my leg. Now I have the lump, and I was stretching more than usual this weekend, which has caused it to be more painful almost like I may have torn something. There is no blood in my stool and I had blood work done, but everything looked good. I am concerned about the lump though. Should I be concerned and could it be just from sitting so much?