Small baby ??

So I went to the doctors last night for a blood test. Whilst I was there I thought I'd just check in with a doctor to ask some questions about the baby. Well I told her that I didnt feel as big as I should be so she measured my belly with a tape measure. She said I was measuring about 33 and a half and I'm actually 36 weeks 1 day so she booked me in for a ultrasound to see if the baby's growing right. Has anyone else felt like this or had this in there pregnancy? Like there been measuring 3 weeks behind and there baby has been fine ?? I'm a bit worried and scared 😳 hope he is measuring right