The Bavarian giants sealed the league crown with victory against Hetha Berlin last month, but they have since failed to win in three attempts.
Saturday night's 3-0 home defeat to second-place Borussia Dortmund condemned Bayern to a second-straight loss in the league for the first time since November 2011.
"It happened to me in Barcelona too that, after we had clinched the league title, we really struggled to raise our game again," Guardiola is quoted as saying by ESPN.
"The aim as coach now is to build the team up again. Maybe I made a mistake. Maybe it was a mistake not just to carry on as we were, but I don't want to make rotation out to be an excuse.
"I think the problem is we've already won the Bundesliga and we've just dropped off. We've got to see things the way they are."
Bayern are back in action on Wednesday night when they face Kaiserslautern in the DFB-Pokal.