There are lots of possible causes and solutions. Some are hard health-related problems you can at best mitigate. But if you're otherwise healthy and just spend a lot of time at the computer, there are potentially some "easy" solutions. In particular:

- Use proper sitting posture, change position often, stand/walk when possible.

- Use an ergonomic keyboard which avoids troublesome positions/movements.

- Exercise. In particular, do some resistance (weight) training.

Anecdotes:

- In my mid 20s emacs pinky could have done me in, but Kinesis ergonomic keyboards worked around emacs afflictions.

- In my late 30s arthritis started troubling me, but a decade later I have less pain (inconsequential) due to taking better care of my health/fitness. But on the downside 16-hour programming days are now a distant memory.

I've read many, many anecdotes over the years, and they make clear that people have diverse afflictions. Be careful about following advice that doesn't apply to your circumstances.