I was wondering where she got those clothes too. lol
So Hata finally explained why Aika was near Athena's Castle's ruins all that time ago.
So I suppose it went something like this. Athena transformed back into a kid and then found a way to change back for a few moments, where Aika probably told her what happened and then Athena went to warn Hayate, where she then turned back into a child, so she could then find a way to return to her original form through proper means (I assume this one is permanent? Or at least it lasts longer).
Meanwhile, I think Aika made more shocked faces these last couple of chapters then she has the entire manga.