What Causes Skipped Heart Beats And Palpitations?

I have had heart palpitation for some time now. I have taken a low dose of Phentermine 15mm. for almost 20 years, only because I was on the Phen Phen diet years ago, and when I went off of it my heart seems to beat very slow, and I was always tired. So my doctor has kept me on this low dose to keep me in a normal state. In the past year, I have gone on diets to loss weight, The Owe diet, which is me taking a higher does of Phentermine or Qusemia, plus an injection of a female hormone. I am now off of the higher does and back on the low dose of Phentermine. My heart is still skipping or palpitating. (I was on the diet for 8 months which I stopped exercising) I am 48 years old and normally active. I should say I have only been off of the higher doses for a week at the most. Do I need to be alarmed and what can happen to me? Thank you, Renee Wilson - Lackey