She would have no reason to change if she hadn't fucked someone else, maybe even more, she certainly had the orgasms she wanted with another man and came back to you satisfied. All for the best, now enjoy your new relationship with her and expect her to be missing from home more often, if she's got a taste for cheating she won't give it up easily.
Don't try to find out what she did and with whom, just make her understand by your behaviour and kind words that she doesn't bother you and has your moral my position support, that you love her even more for being such a slut.