Baby is a little small at 32 Weeks

Rebecca
I just got home from my 32 weeks check up. My OB told me that baby is measuring a little small, and she is going to order an ultrasound to check and see if there is a specific reason behind it. She mentioned that sometimes smaller people have smaller babies (I'm thin), and that her own baby was small.
 Of course, I'm freaking out. Like- is there something wrong? Have I not been eating enough? I've gained the requisite amount of weight. I just want to have this ultrasound so I can get some answers!!