Hi there bri21, and welcome to Scarleteen!
To answer your question, there's always a small chance that you might still get pregnant after taking Plan B, because it isn't a guarantee, but if it's taken within 24 hours, it can be up to 98% effective.
The exact science isn't entirely clear - while it's true that it's marketed as primarily working by preventing/delaying ovulation, there are some hypotheses that it also affects the movement of sperm and/or the fertilized egg's ability to implant into the uterine wall. There isn't a whole lot of research to back this up - for more details you can check out the second section in our
Birth Control Bingo article about it.
So if we think this through, if you last ovulated on February 26th, the days where you would be most likely to become pregnant if you had sex would be a few days beforehand (because sperm can live in the fallopian tubes for a few days), on the 26th, and for about a day afterward (because the egg takes about 24 hours to make its way through the fallopian tubes and uterus if it's not fertilized). So, sex on the 2nd would fall outside of that window of high fertility, so the risk of pregnancy would already be lower, even without any non-ovulation effects from taking Plan B. Does that make sense?
I'm glad to hear that you track your period using an app - that data can help to keep track of cycles and let you know if there's something that's off. That being said, because Plan B is a dose of hormones that impact your reproductive cycle, your cycle will most likely be thrown off for a month or two. You may not get your period on the expected day, simply because your menstrual "clock" was essentially reset by the Plan B.
While we're on the subject, you mentioned that the condom broke. Do you have any ideas why that happened? Also, have you thought about using another method of birth control to back them up in case this happens again?