That's a tricky situation. This is just my opinion but it sounds like she was feeling guilty about something. Before you start thinking the worst it could have been something really small but that she thought would bother you. Maybe the next day she realized that it still may bother you but it wasn't as big of a deal as she thought or that if she told you she would lose you.

When a person is feeling guilty I have found that they try to bring it up and then when it gets difficult try to backpeddle a bit. The first thing I would try to do (and believe me it's difficult), is stop obsessing. The other thing I tend to do is go over the worst scenarios in my head and see how I would handle them. Even if she did the worst possible thing and made a huge mistake would you end it with her? If the answer is yes than I would sit down and tell her how you feel about it without becoming too emotional. Just tell her (face to face), that it's really bothering you and that even if she thinks it's not important, it's important for you to know what she was talking about. It's a lot harder for a person to be apathetic about a situation when they are looking you in the eye. Even if the answer is that you would love her regardless, I would still sit her down and tell her just that. That no matter what happened you would never leave her and you realize that people make mistakes.

It's a hard situation because you can't force a person to tell you something they don't want and by bringing it up repeatedly on the phone she may feel that you are trying to push her away or that you are thinking the worst of her.

I'm not sure how much I helped but good luck!