Hi Hummingbird,
Something I've found that helps a lot is to run your hand under hot water (as hot as you can stand it) for a minute or two when your hand starts getting tight and cramped. It loosens up the muscles and gets rid of the pain.
I guess the other thing to keep in mind is that the first few years of playing guitar are kinda painful no matter what. Keep playing and the callouses and muscle strength will come.
Bryant