This is an interesting muscle and we don’t realize that we are using it.
Do you have a shoulder that is always able to keep a bag, backpack, bra strap, etc on it, without the item slipping off? This is because of a muscle that shrugs the shoulder, the levator scapula. Our bodies are good at not overworking. When you consistently do a specific motion, the body will adapt by shortening some muscles and elongating the opposing muscles. In this case your levator scapula has shortened a bit. How do you help readjust this? Carry your bags differently. Change the load and the way you carry things. Wear clothing that fits well.
Personal Experience: For many years I experienced pain on the right side of my neck by the afternoon. I never understood why. That is until I was forced to stop wearing a bra for a month. (My ribs were too sore to have any pressure on them, from the Heimlich maneuver that dislodged the steak I had been choking on.) When I started wearing the bra again I noticed the pain on the side of my neck, it had disappeared completely when I stopped wearing the bra. I realized that I was elevating my right shoulder to keep my bra strap on. I would find myself for months, after stopping wearing my bra, consciously lowering my shoulder.
Make sure you watch this short instructional video below!