Possibly a miscarriage?

I've been thinking of the worst, so thought I'd ask if anyone has experienced similar,

2 weeks ago I had my first ultrasound which measured bub to be 5 w 3 d a week under what I initially thought, I went for a follow up ultrasound yesterday (2 weeks exactly after) and baby measured 6weeks1 day, the technician was unsure if she could see a heart beat, I've had no bleeding or pain so was a shock, I've had bloods taken today and more in 3 days . Anyone been through this and what was your outcome?