Docs, why do i have a fast heart beat, my resting hb is 100-107 when relaxed and sitting down and ranges from 120-140 while working..

Several posssibiliti: You need to be evaluated. Typical causes are anemia (like from heavy periods), excess thyroid hormone, fever, infection, stress, anxiety, pain, and others. There are also cardiac causes: cardiomyopathy, inappropriate sinus tachycardia, postural orthostatic tachycardia syndrome, intracardiac shunt - to name a few. An ekg, echo, chest x-ray, tsh, cbc, and holter would start the work up.