Tom Taiwo had given Hibs the lead against the run of play before Michael Higdon, James McFadden, Kallum Higginbotham and Tom Hateley scored after the break during Motherwell's 4-1 win.
Fenlon admitted: "I am very disappointed but we deserved nothing from the game.
"We were probably lucky to be in the lead at half-time. In the second half we were very poor and McFadden very good.
"But we knew Motherwell would come at us at the start of the second half. To concede three goals in 15 minutes should not happen with the experienced players we have.
"We still have to apply ourselves in the next two games if we want to make the top six.
"We didn't do enough - we were not good enough."