First appointment

I had my first appointment tonight - according to my last period, my due date should be July 18th but they weren't able to see a heartbeat on the ultrasound and said I'm measuring a little over 6 weeks. My doctor said she's not "too" concerned and it's good that there is a sack. She wants me to go back next week for another u/s. My blood work was done at the end of last week and that came back positive. Has this happened to anyone? I haven't had any cramping, no bleeding, etc so I didn't see this coming so I'm in shock and obviously upset about it.