Everyone wants answers, I still want mine from girls I dated years ago.
At any rate, her behavior indicates she's not a good girlfriend, so you're better off without her. As you move along you'll realize this.
And know that had you never known about her current situation, all this wouldn't be "eating away at you". Certain things in life are best kept as secrets. This is why NC is important.
But to answer your question, if you think she was cheating then she probably was, and her defensive answer to your question confirms it.
