First ultrasound

Amber • Have a 2 year old diva and pregnant with diva number 2!😍❤
Today I had to have an ultrasound just to check up. Well from the first day of my last period my doctor guesses I'm around 6 weeks 1 day. But when I took the clear blue estimator test, it estimates around 3 weeks. At the utrasound today we could only see a small sac they asked me to come in next week....is this normal?