Heart rate keeps changing!?

I am completely relaxed. Not over weight. Don't eat greasy/fat/fast food. My heart was going at a normal rate and then suddenly sped up. I was just sitting upright. It speeds up for no reason and I feel myself losing breath as if I had been running. Then it goes back to normal. But it keeps happening about 1-2 minutes later. I feel on my chest my heart going faster. Whats wrong?