Low fundal height

I had my 37 week appointment today. Everything with my pregnancy has been pretty routine with the exception that I am 35 and Advanced Maternal Age. Because I am “old” they’ve monitored baby girl’s growth pretty closely and I had an ultrasound in the last 3 weeks. My fundal height has been spot on including my appointment last week. At my appointment today they said I’m measuring 4 cm behind (33 vs. 37) and that I need an ultrasound 1st thing in the morning. Has anyone had this before? Playing the waiting game is excruciating. Perusing the Internet is dangerous. Any tales of previous experience would be helpful! Thanks!