Measuring small

So I went in for my check up and my doctor told me I am measuring at 31 wks when 2 wks ago I was measuring at 32.... That's a big difference 😔 so I took a stress test and that came out normal now I am going to get an ultrasound tomorrow to check if baby is actually small. I'm so scared and worried! I know she has dropped since then but I don't know if that would cause so much change. Has this happened to anyone???