What Causes Delayed Periods While On Contraceptive Pills?

Hi,am using contraceptive pills to avoid pregnency...i started to take those from 1 month before..it is a course of 24pills.i took the first pill at the 5th day of my period and continued to take till last tablet.....i completed that pills on 22nd of may.last month i got my periods at 24th april.but now in the middle or the days, sometimes i saw bleeding like 2 drops...till nw i didn't get my periods...so can u pls tell me what will be the problem????