Danger said:
She had sex with him because she wanted to have sex with him. She didn't give two $hits about your thoughts on the matter. She still doesn't care, aside from what it may cost her with you.
This.
Also, don't assume that she feels bad or shameful - moreso not wanting to feel guilty about it. That's why they reveal these things after some time has passed, and try to downplay their bad behaviors at the time. It's on their minds, and they feel guilty about it, and want to rid their minds of that guilt. But they want to make sure they don't have to suffer the consequences for it. Hence, claiming you weren't exclusive at the time, and seeing if she still had feelings for him. This is called BS. There's always an excuse and justifiable reason - AFTER the fact. At the time, she just went ahead and did it - without a damn what you thought. Or even told you about it. Because she didn't CARE - and probably still doesn't.
She likely did the same thing with him - telling him you're not officially together or exclusive, are on the rocks, she's not sure how she feels about YOU, or whatever. This is how they justify cheating, sleeping around, or whatever. The same excuses and lies are made to both parties - with the intention of not feeling any guilt, being justified, and getting away scott-free.
Chances are, if you call her out on it - she'll just call you a jerk, say you weren't even together, and make you look like the bad one. Blah blah. Been there.
Never assume that anything they do is for you, out of genuine "care" for your feelings. In this case, not feeling guilty, justifying their actions, and not wanting to lose you as a result... All for HER.
Even reading between the lines... She's basically saying she needed to know if you were the right one for her - by sleeping with someone else. Not only is that retarded female logic and a bold-faced lie, but she's suggesting that you weren't good enough for her, and she needed to see if she liked her ex more. She didn't need to see if she still had feelings for him, or cared about yours. That's just an excuse and fog of air to be perceived as righteous and justified as she's having sex with him. And that's all it is, when you cut through the BS. It doesn't make sense or add up, because it's NOT TRUE. Feelings have **** to do with it.
Best case scenerio: she wanted to have sex with him one last time. But that's all it was, and should be viewed as. Don't believe any of the fluff inbetween.