I didn't have the exact same experience, but did have something similar. I had an early ultrasound at 6 weeks 6 days when I got pregnant after my loss. The baby was only measuring 5+6. However, I had told the tech to expect that before we even started the scan because I knew that I always ovulated a week later than normal, so I knew the baby would be measuring small. I'm now 15+3 and we've had two more healthy ultrasounds and heard the heartbeat yesterday.
Do you know when you ovulate? Things measuring small can be a bad sign, it can just mean that baby is fine but growing a little slowly, or it can mean that you ovulate later than normal and baby is right on track for when you ovulated.
Fingers crossed for you that it's one of the second two!!