The actress, who plays JJ, was controversially dropped from the show last June.
However, earlier this month it emerged that she has signed a new deal to return as a regular next season.
"We made the deal [April 15], but there have been whisperings for a while," Cook told TV Guide. "The whole thing was so surreal, from being let go to getting asked to come back for two episodes, and then coming back again to do [Paget Brewster's] last episode. I keep trying to have closure on this but it keeps sneaking back in my life."
Cook admitted that being dropped from the show last year felt like "a breakup with a guy".
"It happened out of nowhere," she said. "And it was an, 'It's not you, it's me' sort of thing. You think, 'What did I do?' It's been a rollercoaster ride but I just had to put one foot forward and keep moving."
Cook also thanked fans, saying: "Their outpouring of support really blew me away. That's definitely the silver lining to being let go. The fans really let me know how they felt, and I guess they really let CBS know too."
Criminal Minds airs on Wednesdays at 9/8c on CBS. Cook will return in the current season's finale.