why does my shoulder hurt?

It feels like the day after a football game. You know - when you spend the whole night (or Saturday afternoon) getting thrown to the ground, and the next day you can't lift your arms high enough to eat breakfast. How I miss those days. But the funny thing is that I didn't play football yesterday. In fact, I didn't do much besides play guitar for a couple church services and then do schoolwork. Maybe I was rockin' just a little too hard. Last night as I was typing stuff for school, I started feeling a pain in my shoulder. I figured it would go away. It didn't. It lingered long enough to make sleep difficult, and its still nagging today.