ST. LOUIS — Fresh off the most lopsided victory in franchise history against the Indianapolis Colts, the New Orleans Saints were expected to roll over the winless St. Louis Rams on Sunday.

It didn’t take long, however, for that inexplicable victory over the Colts to be offset by an even more inexplicable loss to the Rams — a team that was 6-21 in the Edward Jones Dome since 2008.

Just like that, a Saints team that couldn’t be stopped a week earlier by one winless team couldn’t get started against another struggling, winless team.

An offense that produced points on its first nine possessions against the Colts didn’t get on the scoreboard until 50 minutes into the game, which, in itself, kind of explains their head-scratching 31-21 loss.

Having 94 total yards at halftime and 130 yards after three quarters is a start. But there was more, lots more, to this one.

The Saints allowed six sacks. They couldn’t run the ball. They had a punt blocked. They couldn’t stop powerful Steven Jackson from running the ball down their throat.

To be fair, the defense did keep the Saints in the game until the final two minutes of the first half, trailing 3-0 to that point.

But it all went south after a blocked punt and interception allowed the Rams, who had scored just five touchdowns in their first six games, to get two in a span of 53 seconds.

So how could this possibly happen?

“It’s not high-school football or college football,” said free safety Malcolm Jenkins, who helped produce the Saints’ first score when he sacked backup quarterback A.J. Feeley and forced a fumble that Jonathan Vilma fell on in the end zone. “Every team is capable of getting a win.

“We definitely didn’t play our best. They have guys, too, guys that are getting paid and are fighting. They weren’t just going to give it to us, so I commend them for the fight that they had and how they jumped on us.”

To weakside linebacker Scott Shanle, it felt an awful lot like two of the Saints’ bad losses in an 11-5 season a year ago: a 30-20 setback at Arizona and a 30-17 home loss two weeks later to a 1-5 Cleveland outfit in the Superdome.

“A loss is a loss in the NFL, but these are the type games you look back on at the end of the season,” Shanle said. “I reference last year in Arizona and losing to Cleveland. Those are the type of games you feel like you should win — and they all add up at the end of the year.”

When asked if the outcome was particularly shocking after they way they played against the Colts in a record-setting 62-7 romp, Sean Payton needed very little time to answer.

“It’s disappointing, it’s disappointing. I’m disappointed,” he said. “It’s disappointing because you take a step in the right direction … you get going and put one of your better games together, and you come back and follow it with a game like this.”