Picky 5 year old

Need advice My 5 year old used to be an amazing eater but recently she has just stopped and is refusing most foods and stopping after a few bites of food. She has had a few cases of gastroenteritis since Christmas and is waiting for a scan on her stomach to make sure everything is ok It's just really worrying me that she's losing weight too fast She was 20kg the other week and has now gone down to 17.9. She says she doesn't feel unwell but just the lack of food is worrying me and I'm encouraging her to eat