Smile, this is a bit of a different situation, but I was abused by my older brother. My mother is also the one who started asking me if I had been sexually abused, and she kept interrogating me about who it was, asking was it this person, that person. She was worried. When she found out it was her (gay) son... well. I heard her call me a liar in conversation. She says I misunderstood, that she was just in disbelief (my brother has since confessed to a T, so it is not in question).
Anyway, I remembered years later that I had told her as a child my brother was abusing me. I was super young, maybe 5. It's hard to say. I could have been younger. But I brought this up to her last year, and she told me that never happened.
The thing is, I know it happened. It did. I have the worst visual memory ever, and I remember every detail now of that night, things I would never have recalled casually, like the color of my mother's bathroom that I regularly forget ever existed in my normal recall.
Anyway, I have come to the conclusion that she blocked this out. My mother suffered her own broken childhood, sexual abuse, and all kinds of crap... I believe she is capable of just blocking out what she didn't want to remember from twenty years ago.