Why hasn't my period started?

I've been recording the distance between cycles for the past few months and it's been 27-28 days. Well since my last period it has been 31 days. I'm not doing anything out of the ordinary. I did have some anti fungal pills for athletes foot, but I don't think that'd affect it. I'm a virgin, and I've had my period for 4 years. Also, I have mild cramps, I guess and light brown discharge. I'm not too worried, but...

How old are you?