Measuring small FTM scared please help

I'm 27w6d I found out yesterday I'm measuring small. My dr didn't tell me how small. My whole pregnancy people have been telling me I'm small for how far along I am. The dr said I shouldn't worry myself but she wanted me to do another ultrasound. My ultrasound is tomorrow! How am I not supposed to worry when it seems she's rushing to do it. Any other mama's have this and have everything turn out ok? I'm sad and I keep thinking I did something wrong to harm my baby.