Hmm, the link that mike101 enclosed doesn't seem to be working.
But I too had red rasied bumps on my arse, right underneath...So sitting was very painful. I thought it was acne, because I know sometimes I get the occasional blemish down there, but these were huge. They weren't all over, but they were very large. They appeared to be filled with fluid. I found out that they were boils. Numerous failed attempts at popping them led me to simply let them heal on their own. It took about a month, but they went away.
Areas that are prone to excessive sweating are good targets for ace. I asked a question in this very forum concerning acne under my armpits, and someone suggested it could be that folliculitis. I suppose it depends on how big these bumps of yours are. Mine were about the size of a pencil eraser.
Maybe I can try to help ya from there, if you still seek help that is. ^-^
if not, then keep us updated on the condition and whether or not it seems to be healing, or if you seek the advice of a dermatologist. I wish you good health, and a bump-free patoot too :3
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Scaredbaka - can sit down painlessly