I need to gain weight

I am 15 5’4 and 83 pounds 😬 I have always been naturally underweight but I should be around 88 pounds right now (I know that’s still underweight but it is healthy for me), but recently I had a really bad flu immediately followed by pneumonia which caused me to loose a lot of weight, it also had a ripple affect on my hunger, this and already having a fast metabolism caused me to loose weight from already being underweight (people always point out how disgusting I look and I cry all the time about it because I just want to look like how I did before I was sick). I am also a competitive dancer and I’m only 3 weeks into the dance season and my ballet teacher and studio owner told my parents that I have to cut classes because I’m too small and I have to gain more muscle. It’s really upsetting because this isn’t really my fault and I was already eating a lot of food, but now I’ve been eating even more food because I really need to gain, any tips on how I can get back to “normal”?