I Hit The Very Back Of My Ankle About A

I hit the very back of my ankle about a week ago and it hurt, but I didn't think too much of it. I went to the gym for two hours the next two days after that. the third day, I could hardly walk. It hurt to step on that foot. I saw a small bruise but nothing big. now after a week, the pain had gotten better but I noticed there's a small ball on the side of my ankle. what is this and should I go to a medic?