4yr old losing weight and not gaining

Hayle • 5 yr old daughter. 1 👼🏼 1-23-19. Ttc baby #2.

So my 4 yr old daughter was a premie. She’s following her own growing chart. 2 months ago she got sick and lost two pounds so her doctor said to come back in a month and see if she gains her weight back. She did. Not more but where she was. 27 lbs. now I asked her doctor if there is something we should give her to up her calorie intake because she’s a picky eater. She was rude and said I created picky eater. Which I don’t believe because when she could eat normal food she used to eat everything we did. Now she wont. She said to make her breakfast lunch and dinner and if she doesn’t eat it she’s not hungry. I tried to tell her she’s stubborn and would go all day not eating then. Well here we are following her orders and she has lost almost two pounds. She weighs 25.8. Any ideas on what to do? She’s my only kid and I know she will be on the small side because of her genetics but I also don’t think she should be losing weight. Obviously what her doctor said isn’t working but she wouldn’t listen to me. Lol I’m at a lost on what to do. Any tips or help would be appreciated.