Pain behind my knee

For the last 3 or 4 week I have been having pain in my knee. This has happend to me before but not as bad were I have to wear a knee brace. It started about Feb I think and than want away. But came back about 4 weeks ago. And not it's to the point were I cant lit my leg that far off the floor with out my knee hurting.  My mom just thinks I pulled something but I dont. Cause this has been on for a while and I have just not said anything. I could use all the advice I could get.