I was going to ask since the memory was "about" you, if maybe you were born hermaphrodite or something. The way you worded what you mother said, I felt like it was "you" rather than something that happened "to" you like sexual abuse that was part of the secret? Maybe you had a twin that got trapped inside of you, that sort of medical thing just becoming known and understood these days but really upsetting back just a few years ago.

It sounds too, like it's not something you are expected to remember? I don't think your mother would have said anything unless it was something that you had no hope of discovering on your own. Do you remember any of the circumstances around when she said it? Was there a doctor's visit or, how did the subject come up? If you aren't aware of anything, you couldn't have raised the subject yourself.

Like Doh said too, it's possible there was some secrecy somewhere, you saw your mother and sister whispering, possibly around the time of the divorce, and you created a story around it to make the divorce and any partially heard conversations make sense to you. I remember how when parents would specifically say, "It's not about you; it's not your fault" those kinds of things, it could often alarm rather than reassure because I wouldn't have thought it was my fault in the first place but now I wasn't so sure :-) There's the whole, "why did they say that?" reaction that children can have because they don't have enough resources to get good information and understanding.