CONFUSED

Okay, so I just found out I was pregnant, well I went to a free clinic where I live before setting up an apt for a test, I am and went back 3 weeks later for a ultra sound and Im so confused, according to my first day of my last period I should be 9 weeks but the ultra sounds tech said according to my “sac” i’m only 7 weeks and was asking are you sure that your last period? I was like YES I track that shit... i was only 4 days late when i got my first BFP what could be the reasoning? i’ve had two children before this and didn’t have this issue!!!