Well, if she did become Ultimecia, we can assume that she had junctioned GF for a lot longer, which could explain total memory loss... Although why she did is another mystery, since after they learned about the memory loss thing the only reason they kept the GF junctioned was because they needed the GF's power to defeat the Sorceress, so why would they keep them junctioned once they've won..?
Sorry, those last two weren't here when I first replied. Gimme a sec, and I'll edit this to respond to them.
Ok, so when Ultimecia possessed Edea, you are claiming that they were still fighting Edea, not Ultimecia, since it's Edea's body? I'm sorry, but I believe that the mind is the crux of a person's individuality. The minds of Rinoa and Ultimecia are those of two different people, even if they came to exist in the same body.