Ultrasound at 39 weeks

So I had my 38 week appointment today & my doctor said she thought the baby is small and wants me to come back Monday for an ultrasound. I didn’t really think people got ultrasounds this late in the game unless something was going on. Waiting 6 days to do it tho doesn’t seem urgent to me. And I thought ultrasounds weren’t really super accurate at measure weight? Anyone else have one so close to their due date because baby seemed small?