it has a lot of buttons on the left side which I use my thumb to make use of. would that be the cause then?
As somebody who used to have a mouse with a side panel - yes, it might have contributed to your condition. Keep in mind that if it’s indeed RSI - it usually develops faaairly slowly over years, and things that contribute to it are on a spectrum. Things that contribute the most are (usually, in our modern setting, but not always) badly chosen mise and phones.
From personal experience - big phones that we tend to have now are very straining on our wrists, especially on our thumbs because we often scroll with a single hand, and doing so on a relatively big screen puts a strain, not to mention that bigger phones are also heavier, and with amount of time we use them per day… hello problems.
I used to have Razer Naga and Samsung A7 2017 in the past, and both combined they did bonkers to my wrist. Naga was just a bit bigger than what I need, as much as I liked its size, and A7 2017 was both huge and heavy, so at some point the pain in my hand and specifically thumb became a norm. I was literally in pain every time I was scrolling a page on my phone. Sounds familiar in any way?
It gradually became much better when I replaced both the mouse and the phone (Razer Viper and Samsung s10e atm) - mostly the phone, as my current one is noticeably smaller and lighter while not having less usable surface (thanks god for the invention of “borderless” screens). I also noticed that my hands get tired much faster if i type on a flat keyboard, so my laptop is always put at an angle on a stand so it would be more comfortable. In regards to game and MMOs - wherever I have a chance, I play on gamepads, because they much less sctraining on hands than keyboard and mouse, especially Dualshock and Dualsense - both are very ergonomic, I can play for hours and feel completely okay after that despite having issues in the past.
That said, look into what devices you use on a daily basis and how much you use them. The phone would be the first thing I advice to pay attention too, with the mouse being right after it. Then I would pay attention to keyboard, as well. Another factors could be how much and with what you write by hand and even what type of physical activited you do. Everything matters.
The most important thing here is to try to remove the main cause of the pain as soon as possible to not let it become worse.