The Bees were beaten 2-0 as Anthony Elanga and Ola Aina struck for the visitors.
Frank remarked afterwards: "I thought we performed well, if I’m honest. I thought we were on top in a lot of moments, but we had more dangerous moments without having clear-cut chances. Defensively, it was probably our best performance of the season.
“Of course we’ve conceded two goals. That’s different. The second goal is obviously a mistake. The first one we can do better too, no doubt about that.
“Maybe we didn’t have enough on the bench because of injuries, but I’m quite pleased with the performance. It happens – that’s football.
“Nottingham Forest have played 4-2-3-1 all season but changed to a 5-4-1 here, which is a credit to what we’ve done."
On losing their home record, he also said: “It’s annoying, but it happens.”