There are a lot of things that could be wrong, but I’m suspecting you’re either straining too much when your lifting or not doing a proper warmup or stretching of that area.
I have always had trouble in the upper trap/scupulothoracic area. Feel around back there with hour hand and see if you can feel an area that is really tight, like a golf ball sized knot of muscle. If you can you may be suited for some trigger point release therapy. This has helped me a lot. Basically it’s a massage that you can perform on your self.
I would search the site for trigger point release therapy, or myofascial release techniques and you should come up with some good resources for you.