fundal height now measuring small

At my 28 week appointment my fundal height measured at exactly 28 weeks (so 28cm). My dr was off the week of my 39 week appointment so I was scheduled in at 31 weeks. When she measured my final height she got a puzzled look and measured again. I measured at 29cm vs the 31 I should have been. I know that 2 cm either direction is normal but she was a little concerned that in 3 weeks I only grew 1 cm. She said at my next appointment in two weeks (so 33weeks) we will measure and if it's off then she will do an ultrasound. I am trying not to worry but I never had this problem with my first. Has anyone else have their fundal height slow greatly in growth? What did they find?