Don't get pissed off at her when she tells you about it. Encourage her. Let her know you find it a real turn-on to be with a slutty woman and you want her to be more slutty. Tell her she can make it up if she wants, and it doesn't have to be true. She's probably afraid you will use it against her, so if she has the fictional option, she might actually telll the truth. If she starts telling you, encourage her, let her know she's turning you on. The next day let her know it meant a lot to you that she shared what she did, and you found it very erotic. Tell her you hope she does it again.
Gee, relax and don't ask questions..she is most likely still having amazing sex with you, so, appreciate her style..
maybe it wasn't so amazing and she'd prefer to keep it to herself