What have I done to my knee?

About 3 months ago my buddy and i were throwing the football and i was running and stopped to turn for the ball and my right knee collapsed.I had swelling and bruising for days,i didnt see a doctor.the swelling went away after 4 or 5 days but I have had constant pain in my knee when it comes to bending it or after I sit for awhile and then get up and walk.I cant use my knee like i use to because everytime i try to, it gets to a certain point and I cant put any pressure on it.Torn Meniscus???Any info will help and should I see a doctor because the pain seems to be getting worse.Thanks.