Ross Draper netted an early goal for Inverness and the hosts wrapped up the points with late strikes from Richie Foran and Billy McKay.
Fenlon found it hard to explain why his side let their standards slip so badly after their recent William Hill Scottish Cup triumph over Hearts.
"It was a poor performance all round from start to finish," he said. "We defended poorly, which we haven't been doing.
"We didn't get to the levels of previous performances. It's hard to explain why straight after the game, we will need to analyse it.
"But when you are not playing well you have to make sure you stay in the game and do enough, but we let in sloppy goals."