What Causes Bump Near Anus After Unprotected Anal Sex?

Hi. I'm a 20 year old male, currently together with a girl of same age, and we've come to greatly care for another. Wanting more from one another, yet fearing pregnancy, we decided to turn to anal intercourse.

We've had the intercourses maybe once every two weeks, never more than once a week. This has been happening for maybe 1.5~2 years. We used lotion as a lubricant, but did not use condoms for discomfort to her. Immediately after an intercourse, we've always thoroughly cleaned the involved parts in a shower.

Recently, maybe 6 weeks ago, she found a strange pimple-like disturbance near her anus, and felt some discomfort. It went away within a few days. Recently, she's got another one, but no pain was involved this time, and when she prodded at it enough, it apparently popped and yielded some liquid.

She didn't feel it itch, and there was no blood. She's apparently gotten the second one after a diarrhea.

What could this possibly be? Could it be caused by anal sex? Would it heal naturally, or should we go see a doctor?

Thank you in advance,

Concerned