Is my girlfriend pregnant?

Me and my girlfriend have been in a pregnancy scaredom for about 3 weeks now. It is pre ejaculation we are worried over. Her period is due around this time period, she hasn't had any pregnancy symptoms so far, however, yesterday, when we were having sex, she started to bleed red, so we were overjoyed, thinking it was her period. However, the bleeding stopped after sex, and she hasnt bled much/at all since yesterday. Im so deathly worried if she IS pregnant, does anyone have any advice??