i have learn't not to take to heart what some people say. i too am a small frame and i think i only put on about 1-2 kg's by 19 weeks, even though i was showing a bit.
people used to say to me i must be having a tiny baby and i got really worried, however now i'm 30 wks and have put on nearly 7 kg's and have this big belly and people are saying to me i must be having a huge baby cause my belly's so big....but i must say i'm only belly, you can't tell i'm pregnant at all from behind, so i just smile and nod, but don't pay any attention.
i've only just now started to put on a few kg's i've noticed each time i visit the dr and i get weighed, so yes it's normal to not put much weight on.