Well, I went to the doctor this morning, and had an ECG. Basically my heart is about as normal as it gets. What had me concerned was that most of the symptoms that I had (besides the shivering) were consistent with a heart attack. Now, I'm way too young to be at risk of a heart attack, but I have heard of enough stories of young people, even pre-teens, getting heart attacks that I was more than a bit worried.
The doctor said that the shivering made him think that I might have caught a fever, but he wasn't too certain.

So maybe I caught a virus at work, or maybe the pain was because I strained my shoulder two days earlier at work and the shivering was just nerves.

I'm getting better so I hope this won't slow me down much.