What Causes Painful Bump On The Anus?

I'm a female, 16, 5'4", and i weigh 144 lbs. I have this sore kind of bump. i don't quite know how to explain it but it's painful when i touch it and it's very firm. i've gotten these types of sores before and i think it may be a result from being molested through anal sex a few years ago when i was 12. Your help is greatly appreciated. Thanks!!