This is an affair and you're being trickle-truthed. She's afraid of the consequences, so she's made up the assault allegations to soften the blame she deserves. Don't be surprised if she comes back and admits to full on intercourse, but tries to say it was rape.
Let's look at the facts here: 1. She meets a new man online 2. Online guy (AP) confesses his feelings for her 3. She makes a secret plan to go visit him, but doesn't tell you despite her always telling you 4. They become physical, but she can't remember 5. GF decides to divulge her affair with AP to you 6. GF claims she was assaulted 7. GF claims she doesn't know what your boundaries are
I don't know how or why you want to stay here