Body help?

okay, im 15 and i jus want to know your opinion and if you can give any advice, i know i shouldnt be too concerned about stuff like this at my age, but i cant help it. ok, im 5 foot 3 inches, and i weight about 46kg (im not sure about pounds, i think its 101/ 102) my belly is ok-ish but its really small when i wake up but gets better through the day. you cant really see my ribs but my hips and elbows are bony. my arms and my legs are thin, although my face is healthy and round. im also pretty flat chested, 32A. i also eat like there's no tomorrow but i dont gain weight, i have a high metabolism. i also havnt had my period yet! is there any advice you can give me on how to get healthier and develop? im pretty behind n my mum was almost complete at around 14/15. i eat quite healthy, but theres junk as well if im being honest. please help me! i dont want to be like this! please give me comforting suggestions and thoughts. thank you, xXx