How can I be pregnant if I used protection.?

IN SEPTEMBER MY B.F && I HAD PROTEXTED .OCTOBER CAME && I STILL DIDN'T GET MY PERIOD. IM STARTING TO THINK IM PREGNANT. IF WE USED PROTECTION WHY IS'NT MY PERIOD HERE.' IM STARTING 2 GET WORRIED. B.C I NEVA MISSED A PERIOD BEFORE.