Birth control pills are not 100% but used correctly they are about 99% effective. With average use the failure rate is about 5 out of 100 women.
The birth control works three ways:
first it can help prevent ovulation
if that fails it also thickens the cervical mucous to make it harder for the sperm to move
if those fail it can cause the lining of the uterus to develope abnormally so that if an egg is fertilized it will not implant.
At the time that you had sex it is not likely (but not impossible) that you were ovulating so there is probably no reason to worry. Ovulation is generally anywhere from 12-16 days into your cycle...The first day of your period is day number 1 and you count from there....But that isn't the same for everyone....It all depends on your cycles. Women can ovulate at any time during the month though so unless you really know your cycles you can't go by that. If you are worried about pregnancy you could also use backup methods such as spermicides or condoms...
Hope that helps.
Hope that helps a little.