My Knee Pops out and really hurts when I bend down?

Since last Tuesday when I bend or kneel down I get this really sharp pain on the inside of my knee and it pops out of place. I don't think I have down anything in particular to hurt it but I am an international competitor in dance and so do a lot of training. Sometime when I try and straighten it again it feels like its stuck and I have to wait a few seconds for it to stop hurting. It is a tiny bit swollen on the inside of my knee but its barely noticeable and when my knees are slightly bent there is a small lump where the pain is but thats it? Anyone else experienced this?