Uhm, it's okay for you to tell her after the fact, but not for her? How is this fair? It's not like she didn't talk to you about her feeling for this guy beforehand. I'm not getting why you feel betrayed, unless it's the fact that she didn't tell you when you shared your activities with Susie. Maybe she truly did forget the entire incident until a couple days later, maybe she felt betrayed because you didn't get permission first...
Personally, I'd give it a pass, mention that you wish she had told you the next morning, then seriously re-think the feasibility of your "tell me beforehand" requirements.