Please go see a cardiologist and do a stress test with EKG. Some offices are now equipped with stationary bikes in addition to the tread mill.

Unless you are uber fit ultra-athlete, low 40's heart rate, doesn't sound quite right. Also have someone else wear your HRM who knows what their HR is and see if it agrees with them. And for you to try theirs out. Compare the result...

And are you drinking lot of herbal tea by any chance? Like hibiscus flower tea? or something else. If you are, check with herbal medicinal books and also tell the cardiologist before the stress test. I sort of remember that the hibiscus flower has an effect on your blood chemistry and heart rate.

smilingcat