I hate to blame everything on MS too but in your case, I agree with Debbie - it sounds like spasticity. I get it there too sometimes and it hurts and is annoying. It does feel like you've ran a marathon when you've only walked maybe 25 feet at the most or, as I call it, it feels like a 1/2 Charley horse that gets stuck like that for hours/days. Stretching the calf muscles out helps. Stand by a wall and do lunges (they help me out).