Fourth-placed Hibs, who play Celtic and Hearts next in successive matches in a true test of their top-six credentials, lost at home for the third time this season.
Admitting his side didn't play at their best, Fenlon said: "We didn't play well today. We didn't deserve to lose, but we didn't set the right tempo from the start.
"We're the home team. We've got to make sure we start the game better than that.
"The emphasis is on us to go and win the game and I didn't think we did that today.
"We looked a little bit flat, maybe waiting for something to happen rather than going and making something happen.
"We only really reacted once they scored. That's not good enough."
After starting the season well, Hibs now have four points from a possible 21.
Fenlon added: "I feel we've stopped a little bit. Getting to the points total we got to should encourage us to see can we kick on and be better.
"That's the key to it now, to try to add to that to make sure we can push on as a team."