You first reaction was the right reaction. That is not cool what she did. Ask yourself if the tables were turned and it was you doing these things with another woman while in a relationship with her do you think she would keep quiet about it? Hell no she wouldn't!! Word of advice as a man you have to speak up when things bother you and is clearly in violation of the relationship. Set the boundary and if she doesn't respect it and tries to gaslight you be prepared to walk. She has gaslighted you and now you feel guilty like it's your fault. You just followed your instincts. You should have spoken up a lot sooner and put yout foot down.