Unless you directly asked your therapist if she thought your husband cheated, or somehow had indicated you thought he did, in my opinion this crosses the line in therapy for her to tell you as much. Did she say "I think he is cheating" or did she phrase it as a question. "He accuses you of cheating. Do you think he might perhaps be cheating?" Even so, it would make me uncomfortable.
For what it is worth, my husband used to constantly accuse me of lying to him or cheating on him. With the icky neighbor across the street, with his crack dealer, with random anonymous people. I wasn't and just yuck. I don't think he was cheating either. Crack makes you paranoid and because I really didn't feel like getting intimate when he was drunk and/or high, I must be getting some somewhere. At least that is what his brain was telling him.
Hang in there.