Ultra sound advice?

So I went for my check up today I’m 35w and 6 days tomorrow I’ll be 36 weeks. Baby girl at our 20 week ultra sound measured within the 20th percentile but so far my uterus was measuring right on track every single week. Today baby girl is now only measuring at 34 weeks instead of 36 weeks so I’m my doctor is sending me for a ultra sound tomorrow morning. Has anyone else experienced this? She said it’s just precautionary as her heart rate sounds good and she’s thinking maybe she’s just sitting lower but it’s making me a little anxious. Ftm here