All good advice. Thanks all! Unfortunately I’m stuck at work all day, so I haven’t gotten any kind of electrolyte supplement, but I’ve been doing lots of stretching, kneading, massaging, and applying heat to it. It seems to be improving very slightly.
I think the most important thing may be to warm up before the gig, and play as sparsely as I can get away with. No need to add fillers, just need to get through the 2 hours alive.
It was the darndest thing. I hadn’t been playing excessively, or death gripping, before hand. The only thing that comes to mind is there was an uncomfortable twinge while I was typing at work two days ago. Maybe that was the snowball that turned into an avalanche.