What happened to my knee?

I kneeled down and my knee went "snap, crackle, pop". It really hurt. Now the pain comes and goes-sometimes it is so bad I can barely walk, and sometimes it's just a dull pain. The pain is on the inner side of my knee and sometimes behind my knee. It's much worse if I kneel down or have been sitting a long time. It's been over a month of this. I'm not overweight, don't over-do exercise and am not ancient! What happened?