She's the ultimate form of sailor moon from a doomed timeline. While finer details aren't clear, she fled the battle with chaos, who was at the time sailor chaos. We don't know how long she drifted alone, but then she decided to go back in time as chibi chibi to stick with the usagi/sailor moon of the main story timeline during the battles with galaxia to make sure our sailor moon didn't make the same mistakes she did.
It's all in the final act of sailor moon, act 52, stars 10.