What is your haemoglobin, potassium, magnesium, calcium, and sodium levels? If anyone of those are off and you are overweight your heart rate goes up faster than it would as if you are overweight alone. Add meditation as part of your training I went to nursing school to and I know what stress will do to you even if you don't think you have any. Oh and check any meds and supplements for ingredients that maybe stimulants your not aware of.