When i tell my doctor my heart is beating fast and he listens with stethoscope, he says it's not beating fast. how can that be?

It depends how fast your heart rate is. If your baseline rate is 60 and when it increases to 90 you may feel it (relatively fast) but the rate is still in normal range and that's what your doctor hears. At times, palpitation may be felt like a fast heart rate or vice versa. Heart auscultation is a reliable way to conclude if your heart rate is fast or not. An EKG tracing is the gold standard.