Are you right handed? I am and my right hand is larger/more muscular and fatigue for the weaker hand might be occurring. Or, with a normal key layout you might have more keystrokes on the left side - have you tried a Dvorak or split keyboard. Finally, is your posture and wrist angle/support the same as for your right hand? I had wrist soreness due to the extreme thinness of a previous apple/Mac keyboard (wrists were bent upwards).

Yes I'm right handed while writing. I've never tried Dvorak because I found it very scary and with a steep learning curve but maybe I can give it a try.

I was also suspecting that the cause might be because of the thinness of the keyboard.

Anyway you have any link that shows the fact that more keystrokes are on the left side? I would love to see the distribution

I vaguely recall the keyboard layout was chosen to slow down typists to keep the letters from hitting each other. Looking down, there are common keys on the left side: asetd…. Just guessing what you’re typing, such as commands or shortcuts.

I think it was more to avoid the arms which hit the paper jamming when two adjacent keys were pressed. I played with a typewriter as a kid and did that all the time.

It's important that your wrists stay straight. If you bend your wrists and type, it causes inflammation of the tendons. So when you see people standing up typing with their wrists bent, let them know it's not a good idea!

If you notice your hand - either hand - starts hurting, my advice would be to rest it. Maybe you just need to take 5-minute breaks more often.