QPR were beaten 1-0 at home by Leicester City on Saturday and followed that up with the setback at the City Ground, where the club have a miserable record.
Goals from Greg Halford and Andy Reid secured the points for fellow promotion hopefuls Forest and Redknapp was unhappy with the performance.
"I'm disappointed tonight," Redknapp told the club's official website. "I thought we were second best in the first half.
"We started the game okay - I didn't think they (Forest) offered an awful lot. But then I thought they were busier than us and we offered no end product.
"We're struggling to score goals and it's a problem. We were without Charlie Austin tonight and Bobby Zamora and Andy Johnson could only complete half a game each because of their fitness, and the injuries that they've had.
"It was difficult but there are no excuses - we've got to do better.
"We've got another tough game at the weekend against Watford - that won't be easy. I'll have to look at the team and maybe juggle one or two around to see what we can do.
"We had a great start to the season. We've had a little bit of a blip now but hopefully that's taught us a lesson.
"We've got to up our game a bit if we want to challenge (for promotion)."
Reid's goal 10 minutes from time prompted controversy after the QPR players stopped when the linesman put his flag up for offside, only for the referee to wave play on.
However, Redknapp said: "I just saw the linesman put his flag up. Then I turned around to speak to Joe Jordan and didn't see what happened after that.
"But at the end of the day, when you first start playing football from six years of age, the first thing you learn is to play to the whistle.
"If the referee doesn't blow the whistle, then you've got to play on. Forest did."