Bump not growing!?

Sara
I'm 35 weeks pregnant. I've had some problems and went to the dr today and the midwife said I have lost weight and belly is measuring 32 weeks so she set me up for a ultrasound next Tuesday. 
I'm worried about my baby. He is very active and always moving so I know he's ok. But what if he isn't gaining weight? What do they do? Anyone experienced this?
It is my first baby!!