One Arm bigger than the other.

Okay so I'm 19 M, 6ft and 49kg type of skinny and I just noticed the hand with I do that with has more shoulder muscles than my other (left) hand, I noticed this when I got pain in my shoulder like the one you get from a workout. And now I'm not sure what to do because if someone notices it they surely gonna know why it is. It is not even like I mastrubate a lot once in 3-4 days. I don't know how it made my muscles grew. Pls help me with some advice.