Man, that's fucked up. I'm sorry you found out like that.
When I got a diagnosis in my twenties, for some time I was convinced that my mother had to know but for some reason never told me. Like, when I recall my childhood, how could you even not notice?! I guess you could if you never heard about autism but I know for a fact that she has at least some idea about it. And teachers, etc...
I don't really believe in this conspiracy anymore. Although I will probably never know for sure because I do not intend to tell her about the diagnosis.