Charlie Sheen -- I Didn't Fall Off the Wagon ... I'm Just a Lightweight

Charlie Sheen is ashamed about his sloshy behavior at a recent Guns N' Roses concert ... but not because he did anything wrong ... it's because he realized he can't handle his booze like he used to.

Sources close to Sheen tell TMZ -- the Warlock realized people were concerned he had fallen off the wagon ... but he is adamant ... he NEVER gave up booze.

Sheen is telling friends he does drink occasionally ... and had a few drinks at the concert ... but they hit him "pretty hard" ... because he simply doesn't have the tolerance he had in his heyday.

Charlie told one friend, "This is what happens when you don't drink very often."

As far as his job goes ... we're told the clip (below) didn't appear to cause any concern with the muckety mucks at Sheen's new show "Anger Management" ... because no one ever mentioned it to Charlie.