This may be due to two reasons. 1. There may be compression of nerve going down to thigh & down at spine, though it is very early as per your age. 2. Weight on Sciatic nerve passing from hip down & behind your buttock & thigh may be pressed due to continuous sitting if either your buttock & thigh muscles are too thin or you are too heavy exerting more pressure in these areas. You must consult a doctor who will investigate & then treat. In the mean time use very soft Cushion under your buttocks when sitting to reduce pressure. Do not bend forward for picking any object from ground sp. heavy one as it may press the nerve & also strain ligaments.