Suddenly measuring behind

I had my 37 week appointment yesterday and fundal height was measuring at 34 weeks. Last week a different midwife measured me 36 and 36 and the appointment before that 33 at 34. There were a few times I measured a week or 2 behind. Midwife isn't worried, but I'm starting to worry because I don't feel like I've grown much belly wise. Ultrasound at 33 weeks showed baby growing fine at 50% estimated weight and fluid levels fine. Debating calling midwife today to see if I can get a scan just to be sure babes is ok. Anyone else have this happen?