What Causes Pain And Swelling In Knee?

I sprained my knee I got xrays I received naproxen to help with the pain and swelling but I still can t bend it all the way I haven t had an mri. It s been 3weeks now I returned back to work.. the pains mainly in the insidethe doctor from work says that s all it is is the sprain I feel that it s something worsethey also said I have a Lil arthritis