I have a bruise on my back right calf just below my knee that has come and gone three or four times now for some number of months with no trauma. It is in the same spot each time, and the other leg does not have any bruising. The bruise itself does not hurt, but there is enough minor tenderness of that area just before the bruise shows that I can now anticipate it forming. It is about a nickle to a quarter in size. Do you know what could be causing it?