Really small amniotic sac??

I got my ultrasound, transvaginal, done last week and measured 7 weeks and 3 days with a heartbeat of 154bpm. It took my OB a minute to actually find the baby which was strange because she never never had an issue finding my first 3 kids this early. Like she had to really press in there just to get a good view. My worry is why is the sac so small compared to the baby? My other ultrasounds in the past with my kids my amniotic sac was always kinda big. Is this normal or okay?