My heart beats fast --why?

LOTS of reasons: Tachycardia (fast heart beat) can be caused by stress, anemia, infection, fever, dehydration, medication, low blood sugar, hyperthyroidism and many many other causes. Some causes are problems with the heart, others cause your normal to beat too fast. Time to call your doctor and get checked out!

Heart rate: Most resting heart rate normals are 80 to 100 bpm. Some people's resting heart rate is higher. Abnormal reasons for higher resting heart rate include anemia, hyperthyroidism, fever, ongoing inflammation or infection, dehydration and there is also a pathologic sinus tachycardia condition. You'd need to see a dr. And be evaluated if you're worried about your resting heart rate.

Tachycardia.: A fast heart beat can have numerous causes and etiologies. If it happens in a moment, and you can figure out why (stress, too much caffeine, got scared by something), then you can wait it out and see if it gets better. If it seems to happen for no reason, then an evaluation by a Cardiologist would be in order to determine etiology.