Any time you have sex there's a chance of pregnancy-- even when you use protection, its just much lower of a chance.

If you're late, take a pregnancy test.

Take a pregnancy test as soon as possible so you know early on whether you are pregnant or not and you have lots of time to decide what to do about it. Other possibilities for late periods can be
-taking birth control irregularly
-eating habits, if you've suddenly been eating far less than normal, or on a different diet, this can affect your cycle
-excercise, lots of excericise can reduce if nto postpone menstruation
-medication, if you are taking any madication and begin to take it irregularly, this can also affect menstruation

BOTTOM LINE: get a pregnancy test! this is the sure fire way to find out!