If a girl has sex before her expected period and then it comes on, she is likely not pregnant. However, some women may have light bleeding or spotting in the early weeks of pregnancy which can be mistaken for a period. If your girlfriend’s cycle was normal with the usual flow for 3-5 days then she is probably okay. Periods are sometimes irregular due to stress among other things so I wouldn’t worry too much about it unless there are other signs as well.