Harry Winks headed the winner in the 93rd minute at Craven Cottage on Sunday, Dele Alli having cancelled out Fernando Llorente's own goal.
Fulham were good value for their 1-0 half-time lead, but defensive fragilities again came back to haunt Ranieri's men, who are seven points from safety with 15 games to play.
Ranieri highlighted decisions from referee Craig Pawson that went against his side, claiming Aleksandar Mitrovic should have been awarded a penalty.
"Unbelievable. Our first half was fantastic and we also had a penalty but the referee didn't see and a penalty is when the referee whistles," the former Leicester City boss told a news conference.
"There was a big penalty on Mitrovic, but it's OK. I think it was a good match, second half we controlled the match very well in a defensive way.
"We gave a gift with the first goal and the second goal is unbelievable to explain because we had free-kicks and I said to players: 'Why do we have to put the ball in the box? Why?'
"We lack experience. There was a ball close to the corner, take the ball to the corner, spend 10 seconds, but the keeper puts a long ball, we intercept the ball and we want to kick on goal.
"[We should] stay calm, keep possession, finish the match and at the end, after three minutes of extra time, they score the goal."
Ranieri has a history of delivering unpredictable results having led Leicester to the 2015-16 Premier League title.
And even though Fulham have conceded a league-high 51 goals this term, the Italian is determined to stay upbeat.
"I am positive because when you play this kind of football, this kind of performance, you must believe, and I believe in my players," he said.
"We are alive, we continue to fight. The team showed fantastic performances. I can only say well done but stay very smart until the end."