What is causing my chest pain?

Last night I started having pressure and pain in my chest that I thought was just heartburn, but I've had it nonstop since then. Occasionally its been spreading out to the sides of my chest and back (mainly the left side) and I can sometimes feel like pressure up by my neck. My heart rate is a little high (around 100-105) and I know that I have SVT so that might have something to do with it. The only thing that makes it feel better is when I slouch or lay down. What could be causing this pain?