What's in my rabbit's bum?

Today, I picked up my rabbit after he cleaned his crotch. I noticed something pink, slightly long and round-ended sticking out of his bum. It slowly retracted back in. I was kind of disgusted by it, and I'm not sure whether or not it can affect my rabbit's health. What is it?

See a vet immediately.
   
It could be a rectal prolapse, which would require surgery to repair.
   
Good luck, hope this works out well.

Eep I sure hope he's fine. Hopefully I can go see a vet soon. I haven't seen it before, nor have I seen it again.
   
Thanks a lot :)