6 weeks but measuring behind.

So I went to the ob today cause I thought I had a uti.. Turns out I don't but they said since I was there they would do a transvaginal ultrasound. So based on my last period I am 6weeks 2 days. He saw my uterus and what looked like a yolk sac, no baby or heartbeat yet... They told me that this means I'm a little earlier than we thought, but did not specify, and said that we would adjust my due date (if needed) at my next appointment in 2 weeks. He reassured me stating he's not worried and said that nothing is wrong I look like everything is great so not to worry. I was just wondering, did anyone measure behind but then at another ultrasound look to be right on schedule or even ahead? Thanks!!