It is not uncommon for a heartbeat to be undetectable during early pregnancy, especially if the gestational age is miscalculated. The upcoming scan will provide more accurate information on the presence of a heartbeat. Many women have experienced similar situations and later found a healthy heartbeat in their subsequent scans. Feeling tired is a common symptom of pregnancy and may not necessarily indicate the presence or absence of a heartbeat. It's best to wait for the results of your next scan and consult with your healthcare provider for further guidance and reassurance.