Very severe pain in shoulder/neck

I started having pain in my shoulder 4 days ago. It was somewhat severe then, but I didn't think much of it, just that I probably slept on it wrong. This happens to me fairly often and results in bad pain, but usually is gone within a day or two. Well, yesterday it escalated into the absolute most severe pain I have ever felt in my life- I almost passed out and threw up, it was so intense. It started going up into my neck then, too.

It was bad enough that I wasn't able to drive myself to the doctor, so I had my dad drive me. I didn't give them much to go off of, considering that I haven't injured myself in any way that I can think of, but they did some X-rays anyways, and they were normal. Then the doctor pressed on my stomach, and asked if that made the pain worse, and it did actually. He asked a few more questions, and then said he thought it was possibly an acute gall bladder attack. They gave me a shot for the pain, and told me if it worked, it was probably a muscular issue. Well, it didn't do anything at all, so they wanted me to go to the hospital for an ultrasound and CAT scan.

I went to the hospital, they did a bunch of tests and blood work etc, everything was perfectly fine, it had nothing to do with my gall bladder at all. They had absolutely no idea what was wrong, but the pain was still extremely severe, so they gave me dilaudid through an IV, which helped for about 5 minutes with the pain, then made me extremely sick.
Since they couldn't do anything else, they sent me home with a prescription for Vicodin, and told me to keep my arm in a sling to help manage the pain. I took 1 and a half Vicodin this morning, which helped quite a bit with the pain, and is still helping.

The pain isn't AS severe right now, but it's still there. But now it's moved under my ribs, and it hurts when I breathe or move my arm certain ways. They want me to go back to my regular doctor in a few days, but I don't know what they are going to do if the hospital couldn't find anything.

What could I have possibly done to my arm that isn't showing up on X-rays or anything? Like I said, this was probably the most severe pain I have ever felt in my life.