What Does Pain And Swelling Around Kneecap While Having Negative Result For Arthritis Mean?

I have pain in both knees for over 1 year, thought it was arthritis. Went to orthopoedic dr. xrays show no arthritis. There is much swelling above, below and around both kneecaps so they won't fall into position. I have edema in both legs which got slightly better after 1.5 wks. of diuretic, now that I have tapered off the swelling is back. Very painful to walk.