Heather knew by what John had said that he’d been spying on her and had seen her disappear into the ladies toilet closely followed by the barman. He couldn’t have known for certain though that she had actually had sex with the man, but she did admit to herself that seemed quite a reasonable conclusion for John to have drawn.
She was now at a crossroads. She had three choices. Continue to deny that anything untoward had gone on with the man, admit and try to explain her actions or say nothing. She chose the second... but maintained her aggression.