Fundal Height stucked at 31 weeks

May nakaexperience ba ng ganito sa mga moms here? During my 33 wk check up, measurement ng fundal height ko was 31 cm lang. Then last wk, 35 wks ako, still at 31 cm. Scheduled ako for an ultrasound tomorrow para sa biometry ulit pero i'm quite concerned. Though maliit naman talaga ako magbuntis based sa 2 anak ko. My eldest was born na maliit, 2.3 kgs lang. 2nd was 3 kgs. Pero i don't remember kasi na nagstall ang fundal height measurements ko.