Usually this happens when blood supply is interrupted for a short time, and is reversible (a condition called paresthesis). It happens when your position causes an artery to kink, like a garden hose,...
Shake your hand vigorously, and stop to massage it with the other hand every few moments. Don't slam your hand into any objects while doing this, and don't shake so vigorously that you hurt yourself....
Has it been exposed to cold temperatures for a long period of time?-possilble frotsbite. Or did you injury it somehow that you can remember? It could also be a pinched nerve. I would go see your...
The condition is likely caused by one of the following:
(i) a reaction to sunburn.
(ii) allergic reaction to the sun cream you're using.
(iii) a mild form of eczema, aggrevated by sunlight.