think of practicing guitar as the same thing as hitting the gym. ur doing the same thing, working the connection between your muscles and the central nervous system, and increasing the fitness of those muscles, the only different is that your hand muscles are smaller than your chest muscles.

you always want to warm up and loosen the joints in your hand before your play. not only will you find yourself to be playing better and more fluid, but it will also help prevent these cramp problems. i know it sounds crazy, but for about 5 mins before i play i stretch most of the muscles in my upper body (yes even ab and back muscles all the way down my arms to my fingers). it helps overall bloodflow to your hand before you play. follow that up with some fingering exercises on the guitar, go from slow to fast (i apologize if for reiterating this if this is already something you do)

go easy with playing for a bit, but dont completely stop. try massaging the muscle thats getting tense, also try expose it to warm-hot water for about 10 mins a few times a day.

hope this helps, ive had things like that happen b4 its a total bummer