Don't tell her. If she won't find out any other way, then don't tell her. It will break her heart and she'll lose trust in you. Don't.
Sometimes ignorance truly is bliss.
Also, I can guarantee that she won't believe 'I didn't mean it'- she'll think you're just trying to make it seem less bad and she will hate you all the more for it.
TELLING HER WILL NOT END WELL. I'm a girl, and I would rather not know if I was in her situation.
Top tip- DON'T DO IT AGAIN.
BUT, if there is any chance that you were seen, please tell her ASAP before someone else.
Good luck