it's not uncommon for your period to be irregular at times especially when you are young. are you more stressed out than usual? have you lost weight recently or have you been exercising a lot? these things can all affect your period. if your period doesn't return in a month or two you might want to talk to a doctor to make sure all is ok.
i'm 16 girl and i've had my period for 2 years. now i havent had it for 4 months. not pregnant i've never had sex.