OB doesn’t know what this is

So I had my first scan at 7 weeks and 1 day and the Dr. saw something in the scan that she seemed confused by. I asked if it was another baby and she said “there’s only one yolk sac so there’s only one baby” when I asked what it was she said it was too small to tell and we have to wait until the next scan in 4 weeks. I can’t stop thinking about what it could be and I’m stressed and nervous. Anyone have a similar experience? 4 weeks is a long time to wait to find out what is going on with my baby.