I have a pain in my left knee when I bend it all put all my weight on just that knee.?

This started about 6-8 weeks ago I was playing football (soccer) i swung to volley the ball and a defender and suddenly got in my way and my leg was extended and I smashed my kneecap at full force against the others players side. At the time for a minute I couldn't walk on it but I limped around for 5 minutes then as we didn't have any substitutes played on for the remaining 40-50 minutes. It has since slowly got a bit better each week but still hurts significantly when bending or when taking part in long periods of exercise. It isn't the front of the knee that hurts its as if when my kneecap took the impact it damaged a part behind there (maybe a muscle) as the sharp/ache pain I get is behind the knee area. If anyone could tell me what they think I've done and how to make it better that would be appreciated.