What to do when this happens?

So my friends had the same problem and fixed it but they wont tell me how. I've been gaming for 8 years now on pc and console, recently i figured out while playing pc my right arm has been getting bigger and i use my left arm the most. This is a serious thing for me and I've started to work out but it feels like its making it worse. I want to keep playing league of legends but this problem is stopping me. anyone that can help me please msg in the comments your recommendations. I just want to play lol all day without worrying.