This has been going on for a few years now.
In my left shoulder I randomly get a feeling like I've trapped a nerve, even if I haven't moved my shoulder. Its so sudden it makes me jump and literally feels like someone has just pinched my shoulder really hard. It doesn't hurt at any other point, and the trapped nerve feeling only lasts for a second or two.
If it helps I've spent the last 5 years at college doing a very practical subject which means I use my arms and shoulders a lot, and obviously I've been carrying around school/college bags for years. If you have any light to shed feel free