Geno tells TMZ ... he left his South Carolina rehab facility after hitting 9 months clean back in June. He says he left in good standing with a certificate of completion -- but regretfully relapsed on alcohol last week.

As for why, Geno says the addict in him told him a drink wouldn't hurt, adding he convinced himself he was doing good -- the fact it was alcohol and not drugs seemed innocent enough to him.

He tells us he hasn't had another drink since last week's incident, and doesn't plan on having one -- but adds he isn't going back to rehab. Instead, Geno says he's going to continue to apply what he learned and continue to attend AA meetings.

As we reported, Geno told us he tried to kill himself last year by chasing 90 blood pressure pills with a liter of Jägermeister ... and says after telling June about the OD, she referred him to the SC rehab center.

He has an active arrest warrant from 2021, after failing to show up to his community corrections program -- part of his sentence in a 2019 crack cocaine case. Macon County Sheriff André Brunson told us while Geno was in rehab, they weren't going to pull him from the program.

Since leaving, the Macon County Court Clerk confirms his criminal case is still active ... and Geno says he's working on a resolution.