What is wrong with the back of my knee?

I was running outside one day and i run pretty fast and i was playing hide and seek with friends and i was being chased so i ran and then i felt pain on my knee, it was on the front and back side. so i stopped for a second then started to run again and after i did that it really hurt i couldn't straighting it then cause when i did it would hurt or when i walked on it.. So i went inside and my mom put some cream on it that heat it up then a ace bandage on it. I went to sleep that night and woke up and on the back of it was hurting. Only when i straighting it it would hurt but not that bad it just was like pressure or something like that but i cant run cause it hurts on the back.. Can someone please tell me what is wrong??