Eric git is also correct, but perhaps this may make things clearer
"
You don�t have proper periods when you are on the pill. What you have is called a �withdrawal bleed�. This bleeding (which doesn�t always happen) is caused by you not taking hormones in the pill free week.
If you took all your pills correctly and you didn�t have an upset stomach or take any other medicines which might have affected the pill, then it is very unlikely you are pregnant. So start your next packet at the right time. If you are worried ask your doctor or nurse for advice, or do a pregnancy test. Always do this if you miss more than one bleed. If you do become pregnant, there is no evidence to show that taking the combined pill harms the baby."
taken from
http://www.fpa.org.uk/guide/contracep/compill. htm#20
what pill are you taking?