Early Ultrasound and Decreased Heartrate

I went on the 29th for an early scan and measured 6 weeks with heartrate of 111bpm. I went back today for my intake appointment and baby had grown as it should, but the heartrate was at 99bpm. The NP that I saw seemed optimistic, but also said it doesn't look great as they aould expect the HR to have increased. She did say the growth was an excellent sign and that sometimes the measurement can be a little off since baby is still so tiny. I'm trying to stay hopeful, but not wanting to be too hopeful that I upset myself more later on at the next scan. I go back in 11 days to see progress. Does anyone have any experience with anything similar and the heartrate decreasing some? I'll be a nervous wreck till my next scan! ** Also I can't go back any sooner due to travel and a lapse in insurance coverage from one week to the next. **