I have pain in one hip that goes down the back of leg. yesterday my knee started to swell and entire leg hurts now. what could this be? no injuries.

We still need histry: There are several possibliities including depending on your age hip arthritis or other hip condition. There is also the possibility of sciatica, pyriformis syndrome, radiculopathy, knee arthritis just to name a few. Thus an evaluation with your primary MD would evaluate your history for peripheral vascular disease, edema or other systemic condition. Then they can refer to Ortho, if appropriate.