Left shoulder+arm is way weaker than the right one

Alright so I am at month two week two and yesterday was Chest, shoulders and triceps. While doing the fly's and Y presses and similar exercises, I realized that my left shoulder is way weaker than my right shoulder. It gets progressively harder to keep my left arm straight during fly's or the triceps extensions as I go further into the workout. I DO NOT feel any sharp pain or anything, so I probably have not done any damage to my shoulder. The only feeling is the tired muscles, that I also feel for the right side, but not as much as the left one. Has anyone ever encountered this problem? Any suggestions will be greatly appreciated.

Thanks !