Tiny pregnant bump? Shouldn't I pop?

Hi, I am about 5'4-5 and have gained 18lbs since my last prenatal last week, and the doc says everything is normal and my baby girl is growing normal. But.. In a few days I will be 28 weeks now and I seriously still have a tiny belly like a 4 months preg person or something. All my sisters were "huge", they really were gigantic and I was looking forward to having a big cute baby bump and instead I have to really exajerate or flaunt my tummy with certain shirts just to get anyone to notice. Otherwise I hear, "I didn't even notice you were preg" alot. Or "wow your really due in a few months". I ume I am carrying more inside because I always have major back pain and my organs are squishing and hurting but I really want her to just pop out. I feel her "all day long" She is quite the hyper little mover in there. It's pretty easy to feel her from the outside too. I was dieting before I was pregnant and lost 20 pounds, got down to 110lb. I was pretty tiny and my family didn't really like it, said I was too skinny. So now that my belly isen't blowing up they tell me that im still too small, "im not feeding her" ect. And I get sad cause I eat like crazy. The docs told me not to listen to them that everything is normal. I just look at babies 28 weeks on the internet and they look huge or how it says she should be like 2 1/2lbs and 15 inches, and I try to envision her in my belly that big and can't imagine she'd fit unless she's literally taking up even the space of my back bone! I wonder if she's just very tiny or what. I hope she's ok, I just thought i'd pop out forever ago.