I Banged My Leg Quite Hard Falling Over. It Hit

I banged my leg quite hard falling over. It hit just below the knee on the inside. I could hardly walk on it for the first day but the pain has subsided and even though I still have some pain I can walk on my leg. I've noticed though my ankle is slightly swollen and aches now. The area where I took the impact is still swollen although this has reduced. why is my ankle now swollen and aches?