Third trimester ultrasound

I was told i wouldn't have another routine ultrasound after my anatomy scan. I am now 35w2d and at my appointment today my dr measured my belly and after a short pause said she wanted an ultrasound at my appointment next week but didn't say why, so now I'm a nervous wreck. Why would they do an ultrasound this late in my pregnancy? He's been head down since i was 30 weeks and has always measured where he was suppose to be....I'm nervous something is wrong and my dr just didn't tell me so i wouldn't stress.