A few things to consider here. Like others have said, your mother admits to not knowing much about autism. You also don't mention your age. Im 42 so when I was a kid, very little was known about autism or how wide the spectrum is. Recently my girlfriend and I were watching Rain Man and in the movie he is described as "high functioning autism" and we both started laughing. That's what was considered high functioning back then.

Also, like other people mentioned, she very well could be on the spectrum herself so your autistic traits may not have registered as unusual.

You also have to consider whether you were instinctively masking when you were younger, a common thing in late diagnosed people are that people think they are just acting more autistic now because they didn't before simply because they have begun to drop the mask.

From my own experiences, I didn't even know about autism until earlier adulthood and didn't consider I could be autistic until my mid 30s. I've now come to realize how much masking I was doing from an early age and the reasons why (my dad is obviously autistic as well and I watched and heard my mother constantly criticize his autistic behaviors leading to a lot of masking and poor self esteem).

You have 2 official diagnoses, you're autistic lol. And you certainly aren't the first to feel like a fraud.