I've seen this question asked on many forums. I'm in the medical field but this is not my area of expertise, I only speak from personal experience. The muscles at the base of the penis are actually contracted when the penis is soft, they only relax fully when it is fully erect. Having an orgasm (oral, masturbation or sex) without the penis being fully erect means those muscles are still tight when the penis spasms, and for some reason this can cause a lot of pain. I have had this problem for over a year and it's so bad sometimes I can't sleep afterwards for hours. Things that have helped me include more time in foreplay to make sure it's fully erect, and trying to pee afterwards. Peeing afterwards hurts but it seems to relax those muscles and the pain goes away relatively quickly.