Baby four weeks and three days. Or cyst??

I had an ultrasound yesterday becuase I was bleeding like my period and passing clots. But there not sure if this is baby or a cyst I say baby. They measured and dated me and I don't know why they did that if I'm not pregnant? Get my bloods back today to see if there is hcg in my system. But they said they can't see a sac or anything. But yet the measures and dated me. Please comment I think it's baby.