Baby measuring a week behind. Need advice

Jordan
Hello fellow moms and moms to be, 
I'm pretty concerned. Today should have marked my 14th week of my first pregnancy. I've had two ultrasounds already and baby measured right on track. Today I went to find out the gender of the baby at an imaging center and the baby measured 13 weeks. What does this mean? Has this happened to anyone? I'm so nervous 😩