What is this lump on my back ?

Im 17, and 3 years ago i went for an x-ray. and since then, i got this lump on my back :o what is thaat ? it is dangerous ? it's not painful. but its kinda big. like a golf ball. its not that hard though, it's like hard jelly. and it's starting to worry me :( i didn't tell anyone about it, not even my parents. it's really obvious when i hunch. help anyone ? im really worried. is it cancer ? D: