i don't know but i got it as well just below the knee is it small and hard?
i believe it could be a condition known as "Osgood Schlatters Disease"
"A small, tender, bony bump may develop a few centimetres below the kneecap. This occurs where the patella ligament attaches to the tibia ('shin bone'). The small bump is permanent, although in time it becomes painless."
taken from patient.co.uk
or you could say osgood schlatters disease/or osteochondritis....inflammation of the tip of the tibia below the knee cap usually from adolescent growth and physical activity such as sports