Is my best friend prego?

Okay so my friend kind of spilled the beans to me that maybe shes possibly pregnant but shes not 100% sure. The one thing that is confusing me is that when she had intercourse, the next day she had gotten her period. Is that even possible? To get your period the day after intercourse and still be pregnant. Because I thought your period flushes everything out? I dont know. Now she feels nausea here and there during the days. Her head is a little foggy and tired. The week after everything happened she has a loss of appetite(loss of weight) and her head feels foggy and tired. She actually feels weak as well a little but that could be lack of food right? So could she be pregnant?