So, apps are good BUT they don't really do anything but look at averages. It's not exact as they don't measure hormones or what is really going on in a woman's body. Cycles work like this: day one of her period is cycle day one. You count from there. Most women ovulate midcycle so the average is day 14. As you can get pregnant a week before that, the fertility window for most women starts on day 9. With that said, any day during a woman's cycle can result in pregnancy if it's unprotected sex. However, you seem to be worrying for nothing. Let me repeat that, you seem to be worrying for nothing. You had protected sex with an intact condom. Pregnancy that occurs with condom usage is because condoms occasionally break. Yours did not. So, she was protected.
Taking plan B would be unnecessary and a bad idea. It's a big dose of hormones which alters your next few cycles. For 2 to 3 months after taking plan b, ladies are often irregular. And in your girlfriends case, this would be completely unnecessary as you didn't' risk pregnancy. Looks like you over reacted and she took it any way. bummer. She now can expect to be late, have spotting, etc. for 2 to 3 months. Expect that. It does NOT mean she is pregnant.
She does not need a pregnancy test. If anxiety such as this will result every time you two have protected sex, I'd say to either abstain OR one or both of you seek some help for undue anxiety. good luck