I sprained my ankle years ago. since, i have had a bump under my ankle on the inside that is hard and sensitive to pressure. why?

Trauma/sprains: Ankle sprains may result in a number of long-term problems. Sometimes, the ligaments will pull off a piece of bone (avulsion fracture) when you are injured, and this may result in long-term sensitivity or pain. Alternatively, you may have ruptured the ligaments around your ankle. If the problem is bothering you, I would recommend seeing a local Podiatrist to have x-rays and further evaluation