Could be many things.

Stress and diet can affect a girl's period. So it could be something as simple as that. Also, some girls are just really irregular.

However, it is also possible she is pregnant. No method of birth control is 100% effective even if it is used correctly. That means that even if you use a condom, and it has no tears or holes, there's still no guarantee that she will not get pregnant.

If she is two weeks late, then she can take a pregnancy test. That is the only way to know sure whether or not she is pregnant.

Either way, she should see an ob-gyn if she doesn't already. If she is not pregnant, she would be able to get another form of birth control to use in addition to the condom if you two want some added protection.