Dortmund have not finished outside the top two in Germany's top flight since 2010, but currently sit 15th having managed only two wins this term.
Jurgen Klopp's men have claimed just one point from their last six outings and suffered a fourth defeat in succession on Saturday when losing 1-0 to Hannover.
Hiroshi Kiyotake grabbed the only goal of the game as Dortmund - not for the first time this season - struggled to convert their dominance into goals.
Hummels told the club's official website: "This is currently a very, very hard phase in the Bundesliga. Everything goes wrong, what can go wrong.
"Slowly it becomes really difficult to find explanations.
"It is the most difficult phase ... for each individual. I believe that you can grow something. But nonetheless, it must be said that what we have planned for this season, is hardly implemented."
Hummels feels Dortmund could have claimed a positive result from every match in their six-game losing run.
"There was not a single team that was stronger than us, but we have taken from six games only a point. This is sheer madness," he added.
