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. 

If the intercourse was rough, it could just be irritation making itself known, or possibly even a small tear in your partner’s vaginal wall. Look out for any sign that it might have been more than that, like if there’s pain at all beyond normal discomfort during sex, or weird discharge afterward. 

• Avoid using perfumed soaps, gels, wipes or other feminine products.
• Don’t douche or wash inside your vagina.
• Avoid wearing tight underwear, leotards, bathing suits or sweaty clothes for long periods of time.

You can get pregnant eve­n a week after your pe­riods. This happens due to ovulation - egg re­lease from ovaries. You might the­n experience­ missed periods, tiredne­ss, and nausea if pregnant. Using protection during intimacy lowe­rs pregnancy risk. 

This is a common side effect of Plan B; nonetheless, there are some cases when it might also cause bleeding. Sometimes, the pill may alter your menstrual cycle and then you'll get the annoying bleeding. But, if you have heavy bleeding, severe pain, or feel dizzy, you must consult the doctor about this. Make it a habit to drink plenty of water and take proper amounts of rest.