Yes, something like this, if true, takes things to a whole new level. You have to decide how you feel about it. For some people, it's a dealbreaker. For others, not so and they can get beyond it. At the very least, it was a mean and spiteful thing for your W to say.
If you are playing games with your W in your communication, just for the sake of trying to get the upper hand, that could be sabotaging your efforts. Why was it a bad time last night and why didn't you want to answer her questions while you had her on the phone?
DB doesn't advocate any gamesmanship, it advocates protect yourself, and work on yourself.
If true, I'm very sorry that it has come to this, I really hope she's bluffing.
You have two options when you talk to her:
1) Ask "would you like to discuss that text you sent me?" and give her the opportunity to expand on the comment. If she asks if you've been dating or had any one-nighters, I would just be honest with her -- "I have been faithful to our marriage".
Just an obvious warning that this will be very difficult to hear, so think through your "escape phrase" to get off the phone before you let emotions take over -- i.e. "I have to process this, I'll have to call you back"
2) Ignore it and don't bring it up at all. The risk here obviously is that it continues to torture you, and/or she continues to taunt you with it whenever she gets upset because it's not out yet.
Lots of thinking for you to do, and a place that nobody wants to be in. If she really has had sex, I'm sure she's hoping you have too so she won't feel so guilty. That may be what's motivating her suspicious comments -- projecting and hoping.
Accuray
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015