A sleeveless base layer can help. I get cold stomach as well. I used to get stomach ache post run. I was told that digestion is slowed right down on a long run as your body concentrates on fueling the muscles and "turns down" non-essentials. Hence a cold tum. I've often wondered if the runners high is something similar, a slight reduction in oxygen to the brain to satisfy the muscles. Hmm. Anyway, you could always try sticking a hot water bottle up your jumper.
Edited: 09/12/2011 at 01:02