I spent three months this year with a cold face - cold upper lip, cold nose, and cold tear duct - on the right side of my face. It was very strange. It felt like I had a chunk of ice on the skin. Unfortunately, it's a paresthesia, so it's either drugs or the heating pad.
How's the color in that foot? I take it you have at least some sensation if it keeps you awake. Is it painful?
I don't know if this would be MS related but it worries me from things I've seen in the past. I think I would get it checked out before the holiday weekend hits just in case it's a circulation problem (like a clot).
PLEASE be VERY careful with that heating pad especially if your sensation is off. You could get a burn or damage tissue in the foot very easily as you sleep.
Mary