I think, kegel exercise may help you, though it is not specific to your problem. As such kegel exercises have many plus points for groin problem. Google kegel exercises and learn the technique and try for some time.d
Sometimes you can tense up so much when you are getting ready to have an orgasm that you can stain a muscle. Try to notice if this is happening to you, it was my problem I would go tense stiff almost like frozen lol just from the sensation of it coming on that after it was over an my muscles relaxed I had bad pain, I had strained my muscles! Maybe what's happening to u sounds like it. Try to relax its hard but with practice you can do it. Hope this helps!
stretching and relaxing is a good work out, if one does irregularly for all parts of your body. One can start with food, calf knee....to top of the head. Stretching one by one and stretching simultaneously and relaxing. You can also identify meuscles you stretch when orgasming, and so this stretching and relaxing as a work out.This will give overall advantage to the body and mind. This can include area around the groin. stretch is to be increased gradually.
There is a book entitles as 'sexersize' which teaches to exercise all muscles involved in sexual activity. Kegel was one of them The book was published in sixties.
Thanks so much for the suggestions!! I thought it might have something to do with that. I'm definitely a freezer lol. Let's see what happens with some modification :D