What Causes Delay In Period?

i am 25 and i haven't had a period in three months, in december i had a brown discharge for like a week and i just assumed it was just my period and in janurary i bleed more days then i didn't but it isn't like my normal period. Well i stoped for a few days and just started doing it again yesterday.. I am really concerened. I cannot be pregent and i am not on the pill but i have taken the pill before.