There can be quite a few reasons for that all of which are a direct result of putting excessive pressure on your shoulder muscles. This is often due in part to your pillow’s inability to keep your spinal alignment in check. If your pillow’s loft fails to properly fill the gap between your shoulder and neck or if it is either too firm or not too firm, chances of you developing a shoulder pain multiplies. Your shoulder can also get hurt as a result of sleeping on the side that experienced some sort of an injury.