Knee is swollen and i Cant bend it all the way back.?

Ok well. In march 2008 i broke my femur. the part that is like two knobs. well i broke it down the middle like right there. i had sury and wore a cast for 2 weeks or whatever. the went to crutches for a month or so. about a month ago me and my dad were messing around and i feel on it and it go all swollen really squishy like. but went back to normal after a week or two. now i started getting into skateboarding again. and i have been ollieing, kickflips, ramps. and going to the skatepark. out of random my knee is doing it again! its not as swollen as it was before but it is puffy and i cant bend it all the way back. whats causing this?