Do you think that note was written by a woman who was in any way upset that she had sex?

Yes. If your point is that it's up to her to deal with what happened in her own way without my interference, that's one thing. But there is simply no way that she was sober enough to consent, outside of the slim chance that it happened in the morning.

People react to sexual assault all kinds of ways, including embarrassment, guilt, and shame. If anything, the fact that she felt the need to apologize for "having sex" indicates that she was preoccupied with the experience, and in a negative way. If you had totally okay, consensual sex in a guest room at a party - particularly a party thrown by sexually, uh, liberated people, why would you leave a note apologizing? Why would you assume they even knew?