What Causes Delayed Menstrual Periods While On Contraceptive Pills?

I am a newly wed and had a baby for 5 months. I've been taking contraceptive pills for 1 and a half month for now but i am still not having my monthly menstration since the time i start taking the pills. I had tried pregnancy test before taking the pill and the result shows i'm not pregnant. what would the possible reason why my menstration is in delay?