| I think it's just cramp as you say. I've had a cramp once while playing bass, though my cramp is in the wrist and bends the hand in towards my arm. I used to have the same cramp when dinghy racing, and it was to do with tiredness.
If you can shake it out, and if the next gig was ok I doubt if it is a big problem: someone with better medical knowledge than me will suggest eating bananas or salt, or something else. | 
