Worried about my daughter

So my daughter is 4 and a half. She was born at 33 weeks but healthy and came home at 35 weeks.

She is severely underweight. She weighs 14kg (2st 2lbs) and she is 102 cm. Despite me feeding her a lot to try improve her weight she keeps losing it. She looks pale and has bags under her eyes. She’s always sleep. And recently she’s been complaining of headaches.

She randomly pukes throughout the night sometimes and will be fine the next day. I am waiting for a doctor to call me back. But does anyone know what this could be? Or if anyone has had any similar situations? Please advise me. Thank you