Detective Nakagawa: It is said in Japan that when a person dies in extreme sorrow or rage, the emotion remains, becoming a stain upon that place. Death becomes a part of that place, killing everything it touches. Once you have become a part of it, it will never let you go.
Karen: There is something evil there.
Detective Nakagawa: Three years ago, three of my colleagues were investigating what happened in that house. Two died mysteriously. The third was never found.
Karen: The whole time I was in that house I felt something was wrong. What happened there?
Kayako Saeki: Croakkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk.
Answer: I don't think she actually went back into the past, I think it was just the house showing her what happened, or at least a type of visual echo of what happened. After all, some pretty strange stuff was happening in that house.
Mad Ade