the pill isn't 100% effective. I was on the pill for 2 years from the age of 14. I got pregnant at 16 whilst I was on the pill. I also caught back before xmas whilst on the pill. The pill is only 9.99% effective do there is still a very very small chance. Its not very often people can catch on the pill but just be carefull if you dont want a child and if Ur worried Ur pregnant take a test.
Well technically while you are on the pill you aren't supposed to have an ovulation week, though some people do. Birth control pills adjust the hormone levels to a) prevent ovulation and b) prevent the lining of the uterus from thickening and thus making conditions hard for an embryo to implant.
It IS possible to get pregnant on the pill, but since they are proven 99% effective, it's not often. The rate of effectiveness also depends on the individual. For some women BCP's work wonderfully, for others they continuously get pregnant on them.
I honestly would NOT worry yourself at this point. Wait and see what happens when it's time for you period.