Baby is measuring small😟

Hi ladies! I am looking for positive outcomes from people who’ve been in the same situation as me. I ovulated on March 1st (confirmed by opks and bbt), which means I should be 7w6d today, so 7w5d when I had my ultrasound yesterday. The thing is, the baby is measuring 6w5d-6d... How can that be? Is something wrong? Could the measurements be off?

I am really stressed out by this because the reason I had an ultrasound is because I’ve been having spotting since last friday. BUT the good news is we saw baby’s little heart flickering away on the screen so I know that’s a good sign! I just know it’s weird baby’s measuring small since I am positive of when I ovulated and did not have sex until I knew I was pregnant.. So if I ovulated a week later than March 1st I would certainly not be pregnant lol.

TIA!