In the last couple of shows I've played, even practice, My fret hand will get really worn out and start cramping up. Mostly when I'm playing chords.

what should I do?
If you have a minute between songs, or a little rest in a song, do that kind of finger roll. It stretches out your fingers. It works for me, at least.
Les Paul Standard
Fender Pro Junior
Epiphone Pr-150
Are you stretching before playing? If not, then this will help a great deal. See the stretching section John Petrucci's in Rock Discipline for more details.