is my bump small?

I'm 28w5d and my doctor has me getting a ultra sound to measure the baby, should I be worried. people say I don't look pregnant at all or that I'm really tiny for 7mnts. I. starting to get concerned I've gained 18 pounds out of the 25 i was told 2. I feel my baby moving and everything, she is a pretty quite baby but she moves some days more then others. I don't want to stress my self out.