How do I get my side to stop hurting?

It started hurting after I slept on it funny three nights ago. I sort of had a feeling something was wrong because my hip hurt while I was laying on it. When I got up, the muscle in my side was really tight-from my hip to my breastbone, and it hurt a lot.
I put one of those Icy Hot patches on it, used an electric hot pad, and put some Tiger Balm on it. Those seemed to help and yesterday it had pretty much stopped hurting.
However, it hurts again today, so much so that I can barely move that side of my body at all. It also hurts when I try to stretch out and lie flat on my back.
What did I do, and how on earth do I fix this?