Open Question: how come my period isnt here by now?!?!?

i am 12 years old i got my period last year...it was an irregular cycle..but it came every two months..or a couple of days late every month....but now it hasnt come in 5 months...is this somehing to be concerned about??....i am not stressed much...not that i know of....is this normal...if so how long will it be before i DO get my period again???......before i SERIOUSLY need to start worrying about it?!?!? serious answers only please!!