Haven't got period

Sarah • Almost 18, Australia, in a relationship (almost 3 years) studying at TAFE.
So I'm a week overdue for my period. there's no way I'm pregnant because my boyfriend always pulls out and I also am on the pill.. I haven't got cramps or any signs of my period except a lot of discharge.. 
Any other suggestions/explanations why I haven't gotton my period yet? Thanks xx