She right. It's not possible. Women ovulate about 14 days before their period. The egg is viable for about two days. If the egg gets fertilized, implantation will happen in about 6-12 days (I believe). If the egg is not fertilized, you'll have a period in about 12 days.
You can bleed during pregnancy, but it is NOT a period, even if it happens on the day you're supposed to get your period. A normal period during pregnancy will result in miscarriage.