you know what, you might also want to look into sciatica, because that actually sounds more like what you have....also called fat wallet syndrome (happens to people who sit/drive for really long times while sitting on large wallets...) and put alot of stress on the piriformis muscle which can press against that nerve...look it up...
doctors will give you all kind of advice about how the muscles gets weak and all that B.S, to be honest with you i think its both, weakness in the muscle and neglect....my personal opinion, warm up before your games, especially if you haven't played for a while, thats what did it for me, i had surgery for ascended testicle when i was little which weakend the muscle in that area, lately i haven't played soccer for a while then i came back hard and played everyday, 2-3hours per day for a week, and to top all that i had to pee really bad in one of the games and i held it and kept playing and thats what did it for me.....bottom line, warm up, and always, always! take a pre-game dump....i cannot stress that last one enough!
see my response to your other post, sounds exactly like what i have, look up inguinal hernia, sypmtoms/early symptoms: (back pain, testicle pain, groin pain, what seems like butt muscle pain especially when you sit for a while, bend over, or warm up before the game, all associated with the same side of the body usually), let me guess, when you play soccer/football you are right footed : ) ? my advise see a hernia specialist as soon as possible because it could get serious....its not that expensive depending on where you live, and if you want to get back to your regular life/sport as soon as possible i suggest doing a laparoscopic surgery, takes one day stay in the hospital or even as an outpatient procedure depending on recovery, and within two to three weeks you'll be back in the game...as opposed to the regular hernia surgery which take forever to heal and comes back after a while AND leave nasty scar like you just had a c-section :)