I can't extend my legs and straighten them. Like if i'm sitting on a chair and I try to lift my legs straight out in the air I can't do it, they stay bent. It feels like my muscles are stiff and if it try too hard later it hurts and burns in the crease of my leg, behind the knee and up the back of my thigh. Also if i'm moving my knees they will crack, grind or pop sometimes. I'm 20 years old, and I think most people can do this, why can't i? My legs just hurt in general a lot, could I already have arthritis?