I have pain in upper knee of left leg only from last 3 days. at first it was mild but from day 3 i could hardly bent my knee&cant put weight on it?

Can be many things: Without a significant trauma, there are several other things to consider. A meniscus tear can cause this type of pain, a stress fracture can make weight bearing painful, gout can cause severe pain and swelling, warmth and difficulty bending and an infection can do the same along with fever, chills and lethargy. I would recommend going to see an orthopaedic surgeon and getting evaluation.