Am i a severely under weight? How the hell?

So i am 17 almost 18 in less than 4months and i weigh 120 & yes i eat alot! so since i was kid i grow up pretty much on fast food micky d's and burger king and am hispanic so i also eat alot of tortias and that sort of stuff at home and to date i have a steady income and i eat ONLY fast food/order pizza & eat junk food i sit on my *** and play video games from 4-9(every single day ive dont this for atleast 1year) i eat a bag of doritos(family size**am not jokin in about 2days mostly once i week cuz am to lazy to go and buy more) i will tell you that every day i have to walk about a mile(not run) to get to school lol so this might be why i dont get fat. But my question is how am a not fat nor do i not gain a single pound i literally eat the most UNHEALTHY food possible my diet literally consist on 3things 2double cheese burgers & 2 large fries,I order pizza+fries,or i get chinese food. Also last thing i DO NOT drink soda i only drink water and i drink about 2 glasses of water per serving i have a habit of taking a drink of water per bite(wierd i know) anything i eat will give me extreme thirst. So yea theres my diet....why the hell am i not gaining any pounds? do i really need to exercise and the opposite of what i think lol?