Only experienced cycling (legs not arms) before where it was definitely a hydration (or rather specifically a lack of electrolytes) issue. Any dietary changes or illness?
Try a magnesium supplement.
Any minor injuries in shoulder/neck/arm in last few years could contribute..
Thinking about it, lots of coffee before climbing and then probably not enough water at the crag can't be helping. Will try harder to drink more fluids...
Forgive my ignorance, but what's wrong with just drinking water?
I might try taking a magnesium citrate tablet or two (i.e. 100-200mg) on days when I know I'll be out doing easy sport for a while and see if it helps.
I use the Nuun ones as I like their flavours, nice but pricey.
I would say that it’s more likely you were asking your forearm muscles to do more than they are currently used.