Chris Kattan -- I'm Not a Drug Addict ... I Just Have a Bad Back

Chris Kattan is not on a prescription pill-induced downward spiral ... he's just got a bad back.

Kattan blew out of LA Friday before the Oscars hit town ... and chalked up his bizarre behavior on a plane -- and his subsequent DUI arrest last month -- to Rx meds after back surgery.

Kattan doesn't tell us when he underwent surgery ... but assures our photog he's doing just fine.

We also asked about DUI charges filed this week, but he's totally in the dark.