Hard Internal Foot Lump

I noticed on the back inner side( between the inner ankle and the back of my left foot) a very hard lump about the size of a quarter that seems to be hard as a rock.
It isnt sore except around it because of the skin stretching and creating a soreness.
I know it isnt from shoe rubbing.
It is on the inside,almost feels as if it is attached to the bone.
What could it be?