She has a chance of being pregnant, anytime you place yourself inside a woman you are giving pregnancy a chance. I can't tell you the odds, but tell her not to stress out cause then her period will be late. Have you guys had sex before this? Or is it a first? She might just be naturally late, not all cycles are on a set schedule for a while. Tell her to wait about 3 weeks and take a test if she thinks she is. But if this happened last night, then I don't think thats why her period is already late