What Causes Pain And Burning Sensation In Calf?

I have pain, aching & burning, in top calf inside of my leg along side my knee but not the knee. I had an ultrasound done last March to check for blood clots which was negative. I can walk reasonably well with very little pain but I can t sit on any type of chair (hard or soft) for very long without terrible pain. I also have this terrible pain when I lie down in bed. I can only sleep sitting up. This is very strange. I m wondering why walking I have very little pain but sitting and laying down is terrible. This all started last Feb. when I started walking daily and had gotten up to 4 miles a day. Then, whammy, the pain started. What happened and when will it stop? I was given a pain medication, Mobick, which was useless. Tylenol actually works a little. Thank-you for any help. Carolyn