you were in your bed with your girlfriend. (You're a girl too) And she was asleep. Suddenly you heard your mom outside. You quickly got outta the bed so your gf would be alone in bed and you sat in your chair. Your mom walks in and sees her sleeping in your bed

Your mom: "who's that?"

You: "that's my friend. She was sleepy and fell asleep"

Your mom: "in your bed?"