Saw heartbeat but said baby looked small

Emily

I’m 8w 6d with my second pregnancy. We had our first appointment today and the midwife did an abdominal ultrasound, warning that we may not be able to see anything yet. She found the baby and we saw a heartbeat. But she said “to my untrained eye (she is not a sonogram technician) the baby looks small for this many weeks. I think you may not be as far along as you think.” This is freaking me out, because I am very sure of the date we conceived. Is there a reason to be concerned if the baby is small? Or should I just be thrilled that we saw the heartbeat? Anyone else have a similar experience at this stage? We have to wait 2.5 more weeks for a vaginal ultrasound and I can’t take this waiting!