I am extremely thin!!!!!! HELP :( :(?

im 17 years old girl, 5'9" and 104 lbs and i have no idea what to do anymore. im gorging on food everyday and instead of gaining pounds, im losing pounds instead. last year, i weighed 135 lbs, and i mysteriously (?) dropped 31 lbs. my 2 year old sister has thicker arms than me. my 8 year old brother beat me in an arm wrestling match as well. and it wasnt that hard for him, because it took him only 3 seconds. last year i was in track but i stopped running track because i felt pain in my knees from weight loss and i pretty much cant do anything anymore. its even hard for me to write long essays and i have to take a rest after every 2 or 3 sentences as my whole arm aches just writing!! i have no idea whats really wrong with me, i dont know why im losing so many pounds even though im getting older :(