I'm not certain this has anything to do with endometriosis, but it could; orgasm generally involves rhythmic contractions of your uterus, and it's been theorized that this could help move stuff out of there (as during your period).
However, it is important that you mention this to your doctor, as we can't examine you and make sure there isn't something potentially serious going on here. It's important that the doctor treating your endo knows what's going on with you, and that involves stuff like this.
It's just part of being responsible for your health.
Kissing girls in English at the back of the stairs, you're a honey with a following of innocent boys, they never know it because you never show it..." - Belle and Sebastian