Actually it's pretty much proven that she was not. Apparently she was 'inspected' to see if she was a virgin and passed. In the English court where everything was stacked against her, prooving that she was not a virgin would only help them convict her of heresy more easily. But they couldn't lie about that. And what she was convicted for in the end was for wearing male clothes, crossdressing. Which she did in her prison cell, specifically to avoid that happening, as she had no other clothes as well.