Pre ejaculation can cause pregnancy. It doesn't matter how much was in, or how long for, it can still be leaked and pregnancy can be a result of this. All you can do is wait and see if your next period shows or not. If not, take a test. You should be using condoms during any sexual acts, not only do they protect against pregnancy, but STD's too.
exactly how big is he ? i am a male and had the same problem with my girlfriend. it ended up just being a hormone thing for her. good luck