Ultra sound

So I was pretty sure I was 6 and a half weeks pregnant! I went to my first ultra sound yesterday and I heard the heart beat at 120bpm, the midwife said its normal. She noted the baby is smaller than what I thought I was she thinks I'm close to 6 weeks but not yet. She asked me to come back next week! I saw the fetus on ultra sound which measured at 0.18 inches, no yolk sack or pole! Should I be worried??