is exposing myself cheating?

so, i have a an amazing girlfriend, at the start of our relationship the only type of privacy i asked for was for her not to snoop through my phone.(i have pictures from the past that i wouldnt be able to stash anywhere but my phone). not complying with that agreement, she found text messages between a FRIEND and i in which we discussed nude picture exchange from 4 months ago in which i sent one but did not receive one.it was nothing more than a TRUTH OR DARE GAME.that was the last time i spoke to her (still within the relationship with my GF). i have only hung out with the other chick once (before my GF) and RARELY ever talk to her.10 days ago, there was a follow up message about it in which i demanded the picture i never received. my girlfriend decides to invade my privacy and finds these messages. she argues that exchanging pics like that is cheating. i explained it was nothing else but a friend thing, which it is, and she disagrees with my point of view.i have never cheated, nor would i, so is that cheating?