My gf is 2 weeks late for her period what does that mean ?

Many things: A late period can mean a lot of things but, if you have been having unprotected intercourse, of course the first thing you think of is pregnancy. She should do a pregnancy test and, if it is negative and her period still hasn't started, repeat it in 3-5 days. If it is still negative she needs to look into other possible causes including stress, weight, diet, medications, thyroid disease, PCOS, etc