Thank you for posting your question.
Adhesions, or scarring from surgery can cause this, as well as inflammation in the pelvis or STD's. Also, hormonal imbalances (low estrogen) can cause this.
If you are having an intense orgasm, it is possible for the abdominal muscles to spasm causing pain also. IUD's can cause strong uterine contraction and painful orgasm also.
The causes can be better targeted by speaking with your gynecologist and discussing the location of the pain, followed by an examination for any underlying problems.
Here is a link for you also:
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