Sometimes when I am laying down or sitting still I notice my heart all the sudden starts beating hard and faster as if I just got through running. While my heart beats as if I was running, my breathing is not faster, it's still the same regular pace. I don't get faint or dizzy, it just doesn't feel good. Once in a while while it happens I take a few deep breaths. I've tried breathing faster in purpose and also slow breathing in through my nose out through my mouth but nothing I do stops it. It stops randomly on its own. Is this lack of oxygen?