Measuring behind

Hi guys, I am very confused here. I found out I was pregnant on September 25th, one months ago( the line was very faint) . According to my periods I am supposed to be closer to 9 weeks.(but I am breastfeeding and my periods are irregular) . I went for my first ultrasound last Monday(vaginal). I was measuring behind at 6weeks and 5days with heat rate 118. They told me that it’s lower then what they wanted. I went for another ultrasound yesterday, at a different place. They did abdominal ultrasound and I measured 7 weeks 1day with a heart rate of 145-160. I feel very nauseous, but I don’t know what’s going on. How can I be measuring to behind every time I go? Did anyone else had anything similar? I am so scared for the baby.