I'm not a doctor or anything but I'll give you my opinion. However you still need to get to a doctor to find out for certain what is going on.
I wouldn't be too worried about the heart from your description especially the part about "it's there pretty regularly. Nothing brings it on, I've noticed it comes on while I'm laying around doing nothing. And physical activity doesn't seem to make it worse."
You might be dealing with a bit of GERDS or some IBS. Anxiety can make both worse.
At your age the biggest question for me would be: Do you have a strong family history of early on-set heart disease? If you do, you definately need a doctor to follow your health history and keep you checked out. Don't mess with a history like that!
Hope you find some answers soon! Good Luck!