Annoying Co-Worker Strikes Again

We all have co-workers who drive us crazy. I certainly have one. But when I go to complain about her, I can never articulate just why she annoys me so much. Here is a perfect example

I was on the phone with a client and I was telling her how I was so sorry that I was late in delivering the names she had requested but that my email had been down on the day that she sent the request.

(We could send emails but not receive them and all the emails that were sent to us on that day were lost forever. We knew this was happening and were not sending any emails internally. My boss had forwarded me the email, knowing that I would not get it. So I really have two annoying co-workers. Back to the first.)

I finish on the phone with the client and go about my business. A few minutes later Annoying Co-Worker IMs me:

Annoying_Co-Worker: is our email down?
E: no
E: not that i know of
Annoying_Co-Worker: i though i heard u say that
Annoying_Co-Worker: no problem
E: no, i was talking about something that happened a month ago
Annoying_Co-Worker: oh

She is in another room! Why is she listening so closely to my conversations? I also wonder why she told me "No problem." Is she forgiving me for having a conversation in my own office?