I Have Had Tests Done In Over Dozen Incidents After

I have had tests done in over dozen incidents after I had a,rapid heart beat they did everything,from ekg to blood tests thyroid etcetera. My heart races after walking up stairs apon walking up at night or it races after doing strenuous activity I sometimes feel short of breath I was told this is all anxiety and it's all in my head. But the heart monitors I wore caught my pulse going up to over 180 after a accidental overdose on Dramamine years ago. I'm tired doctors telling me the same things and I know something is up could it be lack of exercise ? I have not exercised over 2 years.