Hands health

You're basically an athlete. Stretch a little before you play. Goes a long way.
Warm-up always. Not like - play scales before you shred but rather very slow deliberate warming up. Play scales in whole notes and don't press down the strings all the way but rather touch them like you might for a harmonic. Do that for a warmup for a week or so and you'd be surprised how often you accidently fret the note. Lesson to be learned? It requires very little pressure to fret notes and we almost always apply too much. Do the same sort of jelly hands thing with your picking hand too. It's all a system and when you play really hard with right hand your left is more likely to follow suit.
Use a little ice during the day. Get raw ice (not an ice pack and not in a bag) and rub it on your wrist over the sink. Then run warm water over your wrists. Then back to the ice. Alternate like that every twenty or thirty seconds until the ice cube melts.
If the pain changes or worsens while you're playing then take a break. Get a little ice or stretch. Go for a walk. Do some ear training. Whatever. Sometimes ibuprofen is good for getting yourself through the acute stages but it has to be a lot and frequently so it's not really a long term fix because you have to take it with food and it can still be bad for your stomach. No good.
********** if it's really a problem then go see a physical therapist. Not a general practice doctor (except for the referral). A general practice doctor will tell you to take ibuprofen and stop playing. That's not really an option for most of us. Also practicing hard after long periods of rest is a really good way to hurt yourself worse so it's better to cut back and work on staying super relaxed and retraining yourself to play loose. A physical therapist on the other hand will help you work out strategies for preventing the problem in the future given the routine of your life and career. Meaning ... they'll help you figure out how to play guitar without the pain as opposed to stopping playing until it goes away.

I don't really know the science behind all of this but I do know that I had to cut my practice time in a quarter and was worried I wouldn't be able to play my recital for several months before I finished and it wouldn't get better. I went to a physical therapist three times and she gave me the icing trick and a handful of stretches and it was like magic. Haven't had trouble since. Comes back from time to time but it's never particularly bad and it always goes away with the stretching or icing and stays away.