Play video content EXCLUSIVE Chris Ramon/GSXRC1300

Scott Weiland is adamant he was NOT high on drugs (including heroin) during a botched performance of the STP classic, "Vasoline," this week ... but admits something was off that night.

Fans were shocked and concerned after Weiland -- who's struggled with substance abuse in the past -- belted out a flat, uninspired version of the song with a dead gaze in his eyes during a performance Tuesday in Texas with his new band The Wildabouts.

We spoke with Weiland's rep who acknowledges it wasn't Scott's best work -- but says it was a "perfect storm" of Scott being tired and having a couple of drinks before the show. The rep says to make things worse, Scott's earpiece went out during the show, making it hard for him to stay on point.