Girls and ASD can be tricky. They are often diagnosed late, if at all because of their ability to "hide" the symptoms. But I wouldn't think you would have such a rapid, sudden change like that. It would have been more gradual.
Honestly, my first thought would be an event. It's known that children who are abused (I don't just mean sexually, it could be anything and come from anyone incl a bully at school) can often start smearing faeces, lose bladder and bowel control etc. Its a means of self protection I would say but I do know it can be common.
I'm not an expert on either topic, I have a son with ASD and a daughter we were considering having assessed for a while but have put on hold. I also know someone (friend of a friend) who had an abused child who did the poo smearing. So take all of what I said as anecdotal and not an expert assessment by any means.