I've been in and out of the hospital and doc with something similar for 4 months. Mine was so bad to the point I couldn't sleep at night. My heart would constantly race, I would get shortness of breath, lightheaded, feel like I was just going to collapse. First it was thought to be just anxiety (Ive been on Zoloft for like 8 years but actually stopped taking it now) it seemed to get better once i stopped the zoloft, so I assumed ok side effects of the Zoloft all of a sudden. Well within 2 weeks it was back again, went to the doc, more xrays,ekg's,bloodwork etc.. everything normal yet these heart problems still existed. Finally after 4 months I was diagnosed with Costocondritous in my chest, stopped working out chest for a few weeks and sure enough, not one issue. Moral of the story, if you think it's your heart, it most likely isn't if your doc has already ruled out potential heart problems. It's probably some muscle spasms,anxiety and or other issues around your chest.