Girl friend may be pregnant

On June 16th I has sex for the first time With my girl friend. I did use a condom. And admittedly tried to put it on backwards first. I was very gentle and tried not to break it. It also was a spermicidal lubed condom. Anyways we had sex and a week or so later we broke up. She tells me that she's been a little sick in the mornings and her breasts are increasing in size. But since she is a teenage girl (15) still going through growth spurts, that could be the reason for her breasts enlarging. Also, she had been partying it up lately late into the night, so that might be why she is sick the next morning right? Then just today (July 14th) she told me she's missed her period. She had her period the day after we had sex. When we had sex I didn't even cum. We had to stop because I needed to sneak back home.. The condom I used was also a thick ribbed condom. And I wasn't even secreting pre - cum when I was putting it on. It seems so impossible for her to be pregnant. What are the chances?