I have had this exact problem for years. I found out last year that it is a pinched siatic nerve in my lower back. This can be cause by an out of place vertebra or like me my lower back muscles are far stronger than my ab muscles and therefor my posture is bad. Try doing some crunches, bicycles and side crunches once or twice a week. Since I started on that I have not had a single problem with it.
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Always be sincere, even when you don't mean it
The Cell Pimp