worried about weight gain.

Okay so this is baby number two and I have noticed since I was 18 weeks after I found out I was having a girl. That I haven't gained any weight, I'm kinda nervous about it and my stomach size hasn't changed much I am now 23 weeks and still the same size as I was. Should I be concerned about this. I eat plenty and she kicks all the time so I do t know what is going on.