The singer abruptly left the highly-popular girl group without explanation and Bunton reveals she was mad at her bandmate for not telling her what she was going through at the time.
"(I was) angry," she tells Stella magazine. "Geri just left. No words. No explanation. Nothing."
Bunton didn't talk to Halliwell for some time after her departure, but they later reconnected and Halliwell opened up about her experience with depression and an eating disorder.
"You realise what it was like for the other person, how they saw it," she adds. "It puts your friendship on a different deeper level."
The group is now preparing for a reunion, but they will once again be down one member as Victoria Adams has decided not to participate.