What Causes Rapid Heart Beat,chest Pain And Dizziness?

Hi my name is david I am 64 years old and in good shape, Irun and lift weights I started taking 81mg asprin and have stopped because I started having very heavy heart beats my pluse rate is still normal (43) but my blood pressure has been dropping it was 125/70 but lately it is down to 90/50. I am having minor short chest pain and light headedness. This condition is causing me a lot of stress, which is not helping, I had a heart murmer at 13 years old I feel great otherwise, any ideas or advice for me thank you