They might be talking about other guys, in which case she might be embarrassed if you knew what she was talking about. In any case, even best friends keep some secrets from each other. I wouldn't care to talk about a lot of things with a guy, no matter if he was a love interest or not... ranging anywhere from other guys to menstrual cycles.
If there are no other signs that she's betrayed your trust or otherwise been a bad friend, just let it go. You'll feel better. If you insist on meddling in her business, you could ruin a surprise party, embarrass her unnecessarily or just ruin your friendship by annoying her and not letting her have her own life.
As an Apsie, I know I want to know everything and be as close to and connected with my friends as possible, but a good NT friendship allows for more freedom than that, as far as I can tell. I've been accused of getting too smothering before, so best to keep a lid on your curiosity.
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I'm just like you, only different. AS Dx 11/19/2010
Hat size: US 8