Measured 2 weeks early. Need advice!

I had my 8 week appt on Monday and ultrasound measured me at 5 week 6 days. We saw the heartbeat, but rate was 85. Tech didn't seem too concerned, said this happens often and dates may be off. I know exactly when my last period was (June 7). Not sure when I ovulated or conceived though. I have another ultrasound scheduled for next Tuesday to gauge where things are. Nurse told me "these things can go either way" but not to worry too much. Umm okay. Easier said than done. I'm an emotional trainwreck!!! Anyone have similar experience? Outcomes? And please say a prayer for my little tadpole!