I've had it happen a few times in the last while. Usually happens when I've been working hard (chain sawing trees for 6 hrs in 35C+ heat). My GP said it could be repetitive stress injury and/or dehydration. She told me to not work so hard and drink more water.
It started after my MI. I had some sort of blockage in my left wrist which cause a temp difference in the hand. Dr's said it would most likely go away as the drugs worked thru my system dissolving the blockage in my heart but it never did.