From ActuallyAutistic Wiki
My favourite thing is when I'm questioning whether I'm really autistic ("What if I just made everything up?!") while literally stimming.
Imposter syndrome is technically when someone who has knowledge on a subject doubts their knowledge because they now realize how little they know on said subject.
This phrase is often used in the autistic community to describe when an autistic doubts they are autistic. This can be because they have been hanging out in autistic spaces (with or without realizing it), and think, "I'm like everyone else."
This can be remedied by going to a stressful space/situation and recognizing that you are autistic.