You can try to ask her how she feels about her diagnosis, what symptoms does she feel suits the diagnosis best for her, did she feel like she had to hide parts of herself before and what parts, are there any symptoms that she doesn't have that are common with autism. Questions to figure out what she thinks the diagnosis means and poke a little into if she feels like she has to put up a front in some way, or if these are just tics she's been hiding and perhaps some natural regression when getting diagnosed.

You can also go for the more direct route, kinda prime her with "I know most autistic people prefer direct communication, can I speak honestly to you even if it may sound a bit harsh?", and then explain that you noticed she has changed her behaviour drastically since being diagnosed and you're worried that she's not being her authentic self but instead trying to fit into some stereotype, especiallywith actinglike she doesn'tunderstandthings that she used to understand.

Or just call attention to it in the moment (when you are alone!) if she's acting dumber than she is, like "you used to be able to solve/understand this, why do you not understand it now?" or "why are you doing that thing with your face? You've never done it before, is everything okay?" or "you're being quite loud right now, can you please adjust the volume?" or whatever the specific thing is that is grating.