Over time our body parts wear out, so it's not at all unusual for these things to happen for no apparent reason. The problem could be in your shoulder or in your neck. You're going to need to see your doctor to find out exactly what's going on and how to get it fixed. The numbness indicates a nerve is impinged somewhere from the nerve's root in the neck or on down to your shoulder. An MRI should settle that question. An injury like this isn't likely to get better on its own, so make an appointment with your doctor today!