Measuring behind and a little worried

Hi ladies, looking for some reassurance. According to my last period, I should be 8 weeks pregnant today. I had an ultrasound 2 weeks ago and they could see the sac but no yolk or fetal pole. They said I was measuring 5 weeks exactly. Today I had another ultrasound and am measuring 6 weeks 1 day. I thought I would be at least 7 weeks, based on the scan I had 2 weeks ago.

The reason I had the early scans was because I had some bleeding, but haven't had any for a week now. Has anyone else had this happen where they are measuring behind? Did it work out OK?