Maybe she's one of those "don't want to give him (you) the satisfaction" types.
Usually, an exposed cheater is livid, and you'll feel their wrath. But about 15-20% of the time, I have seen what you're experiencing. I think it's some combination of what you said (still don't want to admit to it/drive it underground) and what I said (don't want to give their betrayed spouse the satisfaction of knowing you've upset them).
