Sounds like Raynaud's. My son has it too. There isn't any medical treatment, just advice to wear gloves and be careful - eg don't climb a ladder or go rock-climbing when it's cold, and always wear gloves when riding a bicycle. According to our GP it's fairly common among teenagers, most of whom grow out of it.
Doctor says it is Reynaud's. so gloves from now on. I've also got to have some extra blood tests to make sure nothing else is going on.
Thanks for the advise guys. Finger still hurts from yesterday. :/