First of all you never know when precum is carrying the one sperm needed to penetrate an egg. It seems like she has an irregular menstral cycle like millions of women. This is why many women take birth control. It would be very wise for her to start taking it, especially if you guys are fooling around. And no condom = unintended pregnancy and/or STI (STD) So birth control would be very beneficial, it does not give you the excuse to fool around being unprotected, but instead it will help her cycles become regular so you both won't be so paranoid nor have a surprise on your hands!! Be safe!