Sometimes the heart beats suddenly very fast what reason is

Sometimes the heart suddenly beats very fast, it may be after strenuous exercise, satiety, excessive tension, anxiety caused by physiological heartbeat accelerated. It may also be myocarditis, angina pectoris, atrial fibrillation, anemia, hyperthyroidism and other diseases caused by, need to go to the hospital to do cardiac ultrasound, ECG examination, diagnosis of specific causes.