Oh boy, GH....sorry to hear about this latest. {{{GH}}} Hugs to you.
My opinion - I think deep down you KNEW it was a PA. This could be the confirmation you can use to your benefit and settle this in your mind once and for all.
Bring the subject up - well, why? Your W will probably deny it because she knows it will hurt you and she does not want to do that. Hurt you.....that is.
As others have stated, if she's irregular, along with the stress, it may be just her way of being sure that the unexpected hasn't happened. Not necessarily because she believed she was. Rather to rule it out in order to clear her slate and move forward with you. I don't view this as anything different than a WA going on vacation with OP....of course they are going to lie about it. Would you confront her about a receipt found to a hotel room?
I may be wrong, but I don't see it as a set-back. Rather use it to help you move forward, GH. Face the deepest fear you have and move ahead. Accept that you may never know all the "facts" around her A. I'm not saying remain in ignorance, but try not to dwell on the bad and focus on the GOOD and all things positive. And try and do something for YOURSELF. Call in sick to work on Monday and go to the beach......sorry I don't have anything better to offer.