Bleeding and no sac

According to my LMP I thought I was 5 weeks 5 days yesterday. I started having brown/pink bleeding that progressed to active red bleeding (enough to wear a light pad). I went to the OB and had an ultrasound and they didn’t see anything. Said I might be earlier than I thought, maybe 4 weeks and some change. Said bleeding could be normal. They drew an HCG level and I will get another one in 48 hours to see if it is rising appropriately. Has this happened to anyone? This is my first pregnancy and I’m so scared I lost the baby 😕