What Causes A Delayed Menstrual Period?

hello mam,
i am 20 yrs old.i had sex on the first day after my 5-day period.after a week or so i hav white discharge with bad smell.rite now my period is delayed by a week.i am tensed that i may b a pregnant.i took modus tablet for faster period.i do not want to be a pregnant at this age.pls mam help me....i am really tensed about what is goin to happen