Pressure is building on Spurs manager Frank, 52, as his side sit down in 11th with just 22 points from their 16 games.
Callum Hudson-Odoi and Ibrahim Sangare combined for all three of Forest’s goals, helping lift Sean Dyche’s side five points clear of the relegation zone.
Speaking after the game, under fire Frank admitted the defeat was a “step backward”, saying: "I think there have been some positives step forward the last three games.
“Today the performance and the result was bad. That was definitely a step backward. A very frustrating afternoon.
"I had a good feeling coming into this game. Today it seems like we were in between things. After 1-0 it seems like we were too disjointed.
“We definitely didn't win enough duels or second balls. And another big thing is we kept losing the ball. That doesn't help win football matches.
"Sometimes you just have a bad day for whatever reason. There are various things. We are dealing with 11 individuals that need to be in sync.
“That didn't happen. And then we made two mistakes for the first two goals."