Could be shin-splints? If it's starting to move up your leg. Worth a try - massage your calf muscle well to loosen it up, this takes the pressure off the front of your shin, then whenever you can kneel down with your feet under your arse, toes pointing behind you, which stretches your shins out a bit. I had this a few years back and doing that really worked, completely got rid of it, and pretty quickly too.