confused

so I went to the er due to cramping pains doctor did ultrasound and blood work leaves were at 19,000 or higher ultrasound came back that there is a baby with a heart beat keep in mind my last period was either the 26th or 28th of September that would put me due around the 3rd or 5th if July and I'd be 5wk and 1 no way the bby has a heart beat by now. . . doctor is telling me no u are at 6 weeks pregnant o.o I am very confused tbh also said I'm on the verge of a miscarriage possible even tho I'm having cramping and no bleeding any thoughts to this I'm confused how I could be 6 wk if I found out exactly at 3 week I was pregnant