I'm a 23 year old guy that's not overweight at all. I don't get a ton of exercise, but I do bike around a bit every now and then. I rode my bike 4.5 miles this morning without much problem. I have had some heart palpitations occasionally in the past, but my doc (2 of them over the last couple years actually) said it's nothing much to worry about it it's just occasional. I'm on blood pressure meds too (family history).
Anyways, I've been having an obnoxious pain from my left shoulder blade near my spine up around the top of my shoulder right next to my neck, and down to the top of my sternum. It definitely feels muscular. I can push on a certain spot on top of my sternum and I can feel it a bit, as well as just moving my shoulder in certain positions, or putting a lot of strain on my pecs (wrenching under a car). I haven't felt any tightness or short of breath stuff at all. I've ignored it for about 2-3 weeks or so thinking I slept on it wrong, pulled it playing bike polo (great fun BTW) or something similar, but it doesn't seem to be going away very fast. However, it does seem to fade a bit after some physical activity. It doesn't really seem like something heart problems would do, but I've heard of it before and wanted to get another opinion or two.