weight gain? should I be concerned?

Hi ladies! I'm currently 21 weeks with my first and haven't gained any weight. At my last appointment (16 weeks) they didn't say anything about it and didn't seem concerned but I've been weighing myself at home since and still haven't gained any weight. My bump is definitely getting bigger and I feel all kinds of movement. I eat regular meals and snack every couple hours. I have an appointment later today so I'll ask them then, but I'm just curious if anyone else has experienced a lack of weight gain and how normal it is.