What Could Cause Delay In Periods?

HI, My name is Humera Shafi.i m 29years old.on 2nd july,2010 i got injection of megesterone for 3 months contraception.my mensus date is 26 or 27 of every month.today is 30th july n i have not got mendturation yet.i m tense because i need contraception and my career doesnot allow me to have another baby.my first baby is jst 7months old.what should i do now?