You haven't done something to deserve secretive cheating.
There might be something going on that led her down that path, but that's still her fault for not bringing it up.
If (other than this) she has been absolutely worth fighting for, then you need to talk through it and figure out how to improve your relationship and communication, which leads to a better sex life.
If the above doesn't improve things, and she wants to sleep with other people just because she feels the urge, then you need to decide if you're okay with that, perhaps even involving yourself.
She hasn't stolen all your money. She hasn't taken away your kids. She hasn't ruined your life. It's sexting with someone.