I occasionally get these in my tricep, very rarely shoulder. It's very annoying when I'm trying to get to sleep. I just assumed it was some sort of vitamin imbalance or something. I actually haven't gotten them recently but I have been drinking more milk. The milk has booze in it so its probably net loss but maybe it helps with this.
A few years back I started getting horrendous cramping in my legs after physical activity. So I stopped physical activity! Actually, my dad mentioned he had the same problem and a multivitamin seemed to get rid of it for him, and since I've been taking them I haven't really had any problems.