Michael Phelps admits it “definitely wasn’t easy” explaining those pot pics to his mom, Debbie, a middle-school principal, usmagazine.com reports.

“Seeing my mom reminded me of how it was the day after I got my DUI, and I swore to myself I’d never do that again,” Phelps — who was arrested in 2004 at age 19 for DUI — told the Baltimore Sun. The swimmer could face criminal charges for the alleged marijuana incident, The State newspaper of Columbia, S.C., reported Tuesday.