Is she supposed to be aware of everything happening around her and to her if I'm writing in the first person?
Yes. But she can be aware and not understand.
She can see and describe things she doesn't understand, but perhaps the reader does. Likewise, she may be involved in conversations, or overhear conversations, that only in retrospect make sense to her.
It isn't just first person, though, in 3rd person limited we often follow a single character throughout the book, and know her thoughts, so a similar issue arises.
One alternative is to engage with a character (or book, or scroll, or legend) that is basically telling her what happened, now that she can understand it, and have her remember her role in it or what she witnessed firsthand.
Otherwise this can require some imagination to figure out. In the end it may not be possible to figure out; you just cannot tell the reader about the past event. But that should be okay, the reader should be immersed in the first person character, and NOT expecting something that character cannot possibly know about! It would be a deal breaker for that kind of reader experience.
Whatever the consequences of this prior event, let it happen, and learn the history later because they happened. Just as we do all the time in real life when we are surprised by something happening due to historical events we knew nothing about.