Is it really bad for a 28 week fetus to weigh only 2.8 pounds?

I had an ultrasound at 28 weeks 5 days and. She said the baby weighs 2.8 lbs, and commented that her tummy seemed a little small. I don't smoke, haven't drank, I haven't even so much as dyed my hair the whole time I've been pregnant. I've steadily gained 5-7 pounds every month so I don't understand why she's small. Also, when I researched what her weight should be for her age it was only a few oz off. Is it really a big deal and something I should be worried about, or are the docotrs just being typical and overdramatic to make more money off my insurance? I have a follow up ultrasound in 2 weeks.