Second ultrasound, no heartbeat

We thought we were 7 weeks at our first one, turns out we were only 4 weeks, 5 days. We made another appointment for 2 weeks later; today. The nurse said the heart develops at 5 weeks, 4 days. According to the measurements of Tater Tot today, we're 6 weeks, 2 days but there's no heartbeat. Tater Tot is way bigger than last time, but the nurse was expecting to see a heartbeat. Is this normal. We don't know what to do.