That sometimes happens to me too, usually after a very long run when I'm super hungry. I've never worried about it at all, I think it's just a physical reaction to a good training session and also being a bit energy depleted. Did you actually feel cold or was it more like non-temperature related tremors? If you weren't cold then maybe a check up with your doctor would be sensible, but if you were just a bit chilly feeling I think it's fairly normal. Next time it happens jump in a hot shower or make a cup of tea. I usually come in from my long runs and wrap up in a huge fleecy dressing gown while I eat some hot food...
Edited: 10/04/2012 at 15:34