Giga is right!
Well, I'm not sure what it's exactly called, but I believe something like that can happen. As far as fixing it, I'm not sure. But what I can say is you should tell your friend to be more gentle! The anus wasn't specifically designed for anal intercorse - so you should be more gentle! Just go and tell him...go up to him immediately and say 'I'm your ass (donkey) not your bull' and then when he looks at you funny go tell him 'that means ride me soft YOU ass'
jk or maybe something like that
But I'm not sure what it's called and I can't tell you how to fix it