Pain in right calf muscle, on and off?

For months now I get on and off pain in my right calf. It feels better when I stretch the calf muscle (point heel out), and when I mage into the muscle, it feels like a lump or knot in there. It doesn't hurt to mage it. My doctor had my leg ultrasounded to check for DVT, and the tech didn't see anything. I know it's not exercise related, and it is made worse by sitting a lot, especially in a hard chair. It's never my left calf, though. Any ideas?