If you can't keep the form the same on both arms, and presuming there's been no previous injury, I'm guessing something's tight and that's throwing you out of whack.
Try some of the upper body stuff here:
And read up on other mobility drills and foam rolling. If you can loosen up the tissue that's probably funkifying your movement, it should improve.
But just because the problem is with the left arm, don't overfocus on that side. If parts along your right side (even low back and glutes) are knotted up, it could effect further up the chain.
It's not uncommon to feel lateral raises or bent laterals in the traps, but it shouldn't really be the main place you're feeling it.
Although, if you're only using 5-10 pounds on the laterals, then it could be an issue of everything needing to catch up. The weird issue is that you are feeling it in different places on different sides of the body, so something isn't working right somewhere.