Hi, I'm sixteen, 5'1 and I'm currently on a weight loss program. I'm at 135 pounds but I am working to my goal of 115. Anyway, for about a year and a half I have experienced tachycardia. It has landed me in the hospital and the doctor several times. It normally at resting is 100 or higher, and when I'm walking it's over 120. It makes me dizzy, faint occasionally, tired, and plain sick. What could be the general causes?