It's happened to me a few times. For me it always happens after we haven't been able to have sex for a while for one reason or another and then we will have GREAT sex and I'll cry. I do feel stupid and sometimes chuckle while crying. Mostly the chuckle is to let Alpha know I am fine. Happy tears. I think the first time it happened it did scare him a bit.
I have looked up the cause before and it is something we cannot control.
Doesn't always have to be happy tears though, I know a couple of times I was just very highly stressed about other things and it was so great an orgasm release that it helped me release the other tension too. It's hard not to feel silly when it's something we can't control, but I try not to.
http://www.associatedcontent.com/art...after_sex.html