My new beefy boyf and I have been giving my poor little anus a real workout lately. For the first time ever, I have been getting sore after a bout of anal sex. But just like your lady, only after I have orgasmed.
Partly I think it is because we do block out discomfort when we are "going for it", and the minute after we get the big rush, we become aware of our bodies again.
Partly though, I think it is because the orgasms during anal sex are such "big ones" that your body just wants to stop immediately.
When I come during oral or vaginal, I am always aware of the spasming in my bum, but its just part of the whole pelvic contraction thing. When I come with a penis in my bottom though, the spasms are just so intense that it is equisite but excruciating at the same time. I think it is because I just want to relax the muscles, but I can't because he is still streching me.
Sometimes the feeling of wanting to poo can be overwhelming and distracting too, even though I know this isn't going to happen.
We have found that if he just stops moving for a few minutes while we catch our breath, it is unlikely that he will go soft, and we can take-off again in 5 to 10 minutes.
I get him to gently massage my back while we wait lying in a spoons position, and as he is about to start again, I get him to half withdraw and add lots more lube. After my little rest, I usually find that I open up eve more, and can take him even deeper.
The other benefit is that he gets more accustomed to how tight and hot it is, and can last a bit longer. The benefit for me is that I can have some more of what I really enjoy, and as soon as my tender period has passed, I can get to work on having another orgasm.
...the build-up to which of course, blocks out any remaining feeling of discomfort.

PS Don't ever confuse discomfort with pain. Discomfort can be worked through; pain means "STOP".

Cheers, xx Jill.