(This is just a guess as I have no experience with anal sex.) If they haven't cleaned themselves out before the sex, I would think this would literally back things up and cause them to be a little constipated. Perhaps using an enema beforehand and using lots of lubrication would help to relieve the problem.

Yesh,
I would agree with deteragram that your gal must ensure that she has cleared out the bowels before engaging in anal sex or there can be a tendency for constipation. I always ensure that I have an enema prior to anal sex to ensure that I am clean down there when the guy puts his hard rod up my butt.
Can discuss more in PM if you like.

Wendy