Pretty scared

I went to the dr yesterday for my 8 week (and 1 day) appt. baby was measuring 6 weeks and we saw a heart beat. It was 100bpm but dr said it was on the low side.  We have been trying for 7 months to get pregnant and this will be our second child. My periods have been irregular the entire time 25-35 days. Dr says I could have just ovulated late. I'm trying to be positive but we got our positive pregnancy test when I was 3 wk 5 days (by my calculations) and that would have only been 1 wk 4 days based on measurements. And I know that isn't right. I go back next week for another US.  Anyone else have similar stories?