Weight gain and overall health, how come this is happening?

hey i am a 15 year old boy and lately i have noticed my stomach getting softer and lik bigger..its odd. i used to have abs about two months ago so its odd. the other day i was at a pool and my girlfriend said i looked pregnant. lately my stomach has gotten really round and my thighs and butt as well. like i get hungrier too and i find it really weird. like i think im gaining weight because my pants and jeans have gotten really snug as well and stomach and the sides of my stomach sort of bulge over a little. ive noticed i get really hungry at night as well.my belly button has also gotten deeper its icky...why have i gotten so fat, i eat healthy.