A defeat against Manchester United - which dashed their chances of claiming the title agains their rivals - was sandwiched between two losses to Liverpool over their Champions League quarterfinal tie.
The Sky Blues responded from the three consecutive defeats by beating Tottenham on Saturday. And with United losing to West Bromwich Albion the following day, City were crowned Premier League Champions.
Walker said: "We've had probably the toughest period, definitely in my career at the club.
"We've gone from doing so well in all competitions to take three losses, and sometimes it felt like it was a relegation battle in there with the mood.
"We're all winners and we want to win and in those games unfortunately we didn't. But we put that right.
"That shows the character and the belief in the standard of players in the dressing room."