The Citizens became the first top-flight English side to reach that mark after defeating Swansea City 3-1 in the FA Cup on Wednesday night.
City sit top of the Premier League, five points ahead of Manchester United with a game in hand, but Guardiola has played down the side's recent winning record.
"We are not [unbeatable]," Guardiola said, according to Sky Sports.
"The 15 games in a row doesn't help us to start 2-0 up before the start of the game. I said many times it is nice and helps us to be where we are in the Carabao Cup, FA Cup, Champions League and especially the Premier League, but no more than that.
"We have to start again tomorrow to do the next one. It is an important week with three strong teams and this is the target [to win all three].
"We must try in this time to recover, see what our opponents [Spurs] will bring, what they have done in the game where we played there [in November], what we have to do to play better and beat them. This is the only concern I have after winning in Swansea."
