I recommend to do something like this every day youtube.com/watch

It's an repetitive restrain injury that you're probably getting. I know if i'm typing quite a lot my hands tend to stiffen up after a while. Taking a break doing something else I find helps. I used to find sometimes just having a hot drink and holding the hot cup will help. Try playing with Playdo or lego, doing something else and it might help lol!

That’s a serious and common problem in the industry, and you should consider some steps to take.

1. Take a long(ish) break. I mean days, or even weeks. During this break do something delicate with your hands, like some creative stuff. Anything that requires you fingers moving and doesn’t involve pressing a lot of keys. Your joints need some motion to get back in business.
2. When you are done with 1. or if you can’t afford it, install some software that forces you to take short breaks during your work. One might argue this will “remove you from The Flow” but the flow is overrated if it’s about your health; your health is above all else. I personally recommend WorkRave (also, the project needs some serious love on many fronts, if you need some F/LOSS projects to contribute to).
3. Drink a lot of water. This is probably the most important on the long term. Not coffee, not soda, not energy drinks. Plain, unflavoured, non-fizzing water. Your whole body, joints included, need it.
4. Do some excercises. You don’t have to do heavy lifting nor anything really serious. Even long walks would do. Just move your body. You can do some small excercises during your short breaks mentioned in 2. In fact, WorkRave helps you with that, too.

Take care of yourself. You can churn a huge amount of good quality code out of yourself if you have to stop working altogether due to health issues.

Many years ago, after another 70+ hour week of coding, my right forearm and hand cramped up and stayed that way for several days. Went to the Dr. and he suggested it was the onset of carpal tunnel syndrome (aka repetitive stress injury) and that I might need surgery.

Instead, I learned about proper typing posture and positioning (it has a lot in common with playing the piano). I taught myself all the keyboard shortcuts I could find and learned how to mouse left-handed.

Here it is many years later and I primarily mouse left-handed (but can easily switch as needed) and I no longer suffer any symptoms of RSI in my hands/wrists/forearms.

I recommend going to a physical therapist that specializes in hands/wrists. They can evaluate and give you a set of exercises tailored to your exact strain/pain which will get you feeling better and can be continued prophylactically, preventing future pain and damage (which could get progressively worse and result in eventual need for surgery - best to nip it in the bud).

tilt your keybaord like pro gamers.

I only use ergonomic keyboards since ~ 8 years. So that's one thing that seems to help also I don't use my phone that much besides reading.

I was lucky enough to never have a problem, I am not sure if it's because all of the ergonomic stuff or because I'm not that susceptible to it. But I know some techniques for guitar players where stretching is a big topic. So I would recommend that too :)

I tend to do some stretching and other hand exercises. Also, I have a pretty soft mechanical keyboard which works great for me, I know that some people hate them, but it feels like typing over feathers.