That could be a number of things, but 2 things come immediately to mind. It could be sciatica, or muscle spasm. Both really suck, but seeing as how we sit a lot we're prone to them. I had a similar problem late last year that was accompanied by hip pain. Turned out that my hip flexor and t-bands were in serious spasm. When muscles go into spasm you feel it where they connect. Deep tissue massage worked it out. As was said before, stretching is good too. When you stretch, try resisting while you're in the stretch. For example: put either leg up on something to stretch your hamstring. When you feel the stretch, resist by pulling your leg down by using your hamstring and hold it for 10 seconds. You'll be surprised how quickly you'll gain.