You ovulate 14 days before your cycle begins. Since you're expecting ur period on the 15th, that means when you had sex, you were probably ovulating at that time. A woman is most fertile during ovulation (when ur ovaries release an egg), and since the condom broke and you were most likely ovulating, there is a greater chance of u being pregnant.

I'm not trying to scare you, but go to ur dr and get the morning after pill asap!!

Check out the sticky posts at the top of the "teen pregnancy" section for more info.

I have to agree with june4life get to your doctor/family planning centre asap.
The morning after pill works upto 72 hours after intercourse so if you had sex morning of 1st then 2nd would be 24hours, 3rd - 48hours, 4th - 72hours.
The sooner it is taken the more chances of it being sucessful.
Unless of course you want this baby...