I've had a soft pimple on my inner right buttcheek for over a year now. What could it be?

It kind of feels like a water blister of some sort. I popped it once and I'm pretty sure it spread but the other ones went away, but I'm not sure if that was the cause of the other bumps. It's been here for over a year now and I'm not sure what to do. It gets itchy after doing anal sex, if that helps anything. Went to my OB/GYN about it and had no luck.

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