Unwanted muscle

Hey, so to start things off I'd like to quickly note that I don't normally post threads to websites like this at all due to my being abit introverted when it comes to posting things that many people will be able to read. With that being said, I've searched far and wide for some sort of resolve or understanding of this and have gotten absolutely nothing for it.

For awhile now my left trapezius muscle (shoulder region) and the right side of my middle back have been like overdeveloped for absolutely no conscious reason. I spent from September to February doing conditioning for baseball with lifting and cardio about four days a week and it seemed abit more evened out, but I've stopped lifting altogether for the past month or so just because it kind of lost it's appeal and I wasn't too happy with the results as I'm not too interested in looking very bulky, but more-so lean since I'm not the most masculine of people.

But despite the fallout, those two areas are still problematic and I don't know whether it's because of my posture or habits, and I don't know why nor how to fix it at all, and it's really upsetting because it's VERY noticeable.Could stress on the shoulders prompt that sort of muscle growth? I'm worried if scoliosis cause that to happen?(as in, would there be chance that I have that?) Who would you recommend going to see and to talk about it?