No weight gain and 28 weeks ;(?

I'll be 28 weeks on Thursday ;) I'm so worried tho in the beginning I lost weight like 7 pounds I had very bad morning sickness then I gained I gained like 4 pounds then lost two pounds I'm 28 weeks and I'm still barfing almost everyday I have had my ultasound and the doctor said she is growing perfect but I need to put some weight on I'm not that big of a person what could be going on? I'm worried. Doctor said at my last appoinment that when she sees me again I need to have put on something and my appointments next week and I know I probabley lost weight what do you think there gana do to me ; /