I am almost 16 with a huge prob!!!?

i used to weight 75 kg. and now am like 48 in just like 3-4 months. imy stomach hearts so bad when i eat any food.... i cant even breath. if i eat like one meal i will gain like 2-3 kgs. its so annoyin..... but i always excercise every day. i go to gym and swimming what should i do to be healthy and fit?