Might be having twins?

So last week when I should’ve been 6 weeks 2 days pregnant I went to the free clinic for an ultrasound but she said I was too early to see anything and I could come back in a week if I wanted to. I went back today (a week later) and she said I am still early (according to LMP I should be 7 weeks 2 days pregnant now). She said she thought she saw two yolk sacs and took pictures to send to the doctor to read them. She said that she was having a hard time measuring heart rate but she thought she could see it beating. Well I end up getting a message later on from her saying the doctor reviewed my scan and he did see two sacs but the bottom one looks like it may be empty and to follow up with my OB. (My OB wouldn’t schedule me until I was 8 weeks and their soonest appointment available isn’t until I’m supposed to be 9 weeks.) Well I’m feeling a little worried, I feel like I should’ve been far enough along to see and hear heart beat already. I’m also worried about if I’ll miscarry one baby (or both) or not. Anyone have anything similar happen? Should I be worried or wait it out cause I could be earlier than my LMP? I plan on calling my OB and telling them about the 2 sacs in case they want to see me sooner tomorrow morning. L