I look like i m pregnant(big belly)

hi guy, i m having a serious problem 

i m 17 n i m in a big problem 

i had irregular periods before and bleed only for 3 day,

at the age of 15 i had sex with my boyfriend plenty of times

now i m 17 i look like i m pregnant(big belly)

pee alot, and have periods only for a day no bleeding at all

only spotting, and i have back pains i tried doing excersice but it doesnt help

plzzz help me.....