Hey Eran, it could be due to a medical issue or improper technique, not sure if you'll find a solution here but maybe if you combine/change a few factors, you'll feel better about it.
1. As Jinkasai said above, you shouldn't be sitting behind your desk 24/7 so constantly doing gaming sessions without taking any breaks can lead to it, I'd recommend taking a short break once an hour and maybe you can even perform some simple muscle exercises for your wrist/hands.
2. Maybe you could try using an ergonomic mouse, might be very uncomfortable to use it but I believe it would be better than using a regular mouse, I know a few people who switched to ergonomic mouse due to having wrist pain.
You can try to Google about it as well, I'm sure many gamers have suffered from it.
