They weren't accusing her of murder at the time, at least as far as she knew. The police were telling her they had 'hard evidence' she was at the cottage at the time, that Raffaele had said she went out that night which she didn't remember at all, and that she must have 'repressed' the memory of the murder, and that they were convinced she was 'protecting someone.' Over and over during the course of about three hours from 10:40 PM or so on November 5th, to 1:45 AM on the 6th. Alternating between yelling and screaming at her and calling her names, to cajoling her to help them find the murderer of her friend. She'd been with police almost every waking hour from the discovery of the murder, 52 of about 90 hours total from the afternoon of the second until she was arrested at 5:45 AM on the sixth, upon which time she started her note.
It might be easier to explain if she'd been on hash when she walked into the
Questura (the police station where she was questioned) like Raffaele was, but there's no evidence of that, nor has it been contended.
It is possible she was hit harder than what I personally suspect, and that perhaps it caused some head trauma, but she didn't seem to suggest that either in her note or on the stand. Possible doesn't become probable in my world just because it makes the case better--Massei should take notes!
If you get an opportunity it might help to watch these
two videos which is (part of) her testimony in English. Later she switches to Italian and you can find the transcripts of that and the previous English testimony as well
here. It also affords a look at what she's really like, and what I mean by just a little offbeat, but basically just a
girl. She'd be entirely normal around here, but some look at her and see a monster. Her quickly changing expressions at times were captured by photographers to produce very unflattering and sinister-looking photographs to help build the myth of 'Foxy Knoxy.' See what you think, if you have a chance.
Here's a rather thorough paper on the concept of an internalized false confession, how they are produced, and what sort of conditions make subjects more vulnerable to them. What Amanda experienced was a rather mild form, some confess to murders and walk the interrogators through an imaginary sequence of events step-by-step of their own volition. She didn't do that, she simply believed for a while she must have been there, after all why would the cops lie about having 'hard evidence' of her at the scene, or Raffaele lie about her going out? Perhaps what she'd been told to imagine was real, and she actually did repress the memory like they said?