It happens. It's more likely a boil (akin to a giant zit) than a fybroid (knotted tissue), but both can be cycling related.
Treat either like the infection that they are, with over the counter benzoyl peroxide products on the boil and general antibacterials (bactrin) in the general area.
Soak yourself in a hot bath a few times a day and try to massage it. Eventually it will soften up and go away, or migrate toward the skin and pop. If it does get worse you might have to get a doc to lance it.

Either way, you anus is going to have a lot more fingers poking around it for the next few weeks.