Curious 4 a friend

Hey!,I have a friend who told me a few days ago,that she had unprotected sex with her BF.The next day she came on her cycle but instead of her getting her normal 5 day flow,it only lasted 3 days.Now she's freaking out thinking she may be prego,I say she's not after doing some online research.But then again I'm not sure I have heard of women being pregnant while on there period so I certainly can't cancel it out.Any feedback,comments or advice would be appreciated.?