Recently, my heart has been beating fast, not short of breath or dizzy but since I have never had this before, I am a bit panicked. Sometimes my heart rate goes up to more than 100 beats / min, sometimes it goes down to 76 beats. I looked online and they said it could be due to lack of water. But even if I drink enough water, my heart is still the same. So the doctor asked me to ask if a fast heart rate is a sign of disease? Is this disease serious? I thank you doctor.