8 weeks, spotting, ultrasound baby measured 5 1/2 -6 weeks?

Here's the deal. I had my period Jan. 11. Got a postive OPK at CD 16 (jan 26) and all my CM dried up the next day. So I know I ovulated that day. around 8-9 DPO I had dull cramps, which I have now taken to mean implantion. I got a BFP on 14 DPO (Feb 9). At 7 weeks 4 days I had the tiniest amount of spotting it freaked me out because Jan. 11 is also the day I miscarried a baby (chemical preg. was told I could try again right away) Anyway I called my OB they had me come get an ultrasound and they found a heart beat and said things with baby look good but the baby is only measuring 5 1/2 weeks - 6 weeks tops. The ultrasound tech said they just may change my due date. But how can that be? The tech talked to the doctor who said he wasn't worried and to just come to my first scheduled appointment at 10 weeks. (2 weeks from now) But what the heck? I know with my last baby I had lots of ultrasounds because of GD but her weight and measurements varied by such a large gap and they were way off from what she was even born at, that I don't really put stock in ultrasound measurements and such. But what do you think.