Unexplained calf pain and swelling

Six months ago my left calf began hurting and swelling. I went to the doctor and she sent me for an ultrasound to check for blood clot. It was negative. Sent me home with pain pills and an anti inflamatory. Off and on since then I have had swelling and pain in the same leg. It hurts whether sitting or standing. Haven't figured out a trigger. It is only the left calf. No redness or warmth to touch. Does anyone have any idea what this could be? (I didnt injire it)