I can't really say anything about the autism, I myself am on strattera for ADHD.
For those meds I can say they have the side effect of making me drowsy/buzzy sometimes. A few hours after taking them. Maybe she has something like that and doesn't associate it enough to talk about it, but somewhere still feels like she needs to refuse meds because something is going on?
From a parental view, my daughter had it recently that she suddenly changed her behaviour because of random words from peers in kindergarten. Managed to explain it to her on my end, but it was still tough with a non-autistic but strong headed girl. I don't know your daughter's circumstances, etc, but such a sudden change can also come from external sources you aren't privy to. Maybe she opened up to a kid that then had a strong anti-med stance from its own upbringing? Maybe she was told somewhere that her parents are trying to make her be someone she isn't - even if it's not true. Maybe she's doing it to fit in with that kid/group? Just possibilities.
I'm not saying it has to be something like that, it could also be something completely random. It's just another two items on the list of things to possibly ask about.