week 6 day 3, no heartbeat

Today went to re-visit gynae but baby still have no heartbeat. Initially cannot find the baby through abdominal ultrasound so doctor opt for vaginal ultrasound. Doctor say baby size is 3mm and still no heartbeat. Slightly smaller and have to re-visit at week 8. She prescribe me some medicine to help prevent miscarriage. I am feeling worried. Is there any signs i need to look out for or be careful.