LeAnn Rimes -- RESURFACES After Checking Into Treatment Center

LeAnn Rimes belted out some tear-jerking tunes last night ... marking her first public appearance since checking herself into a treatment center last month.

The country singer took a break from her 30-day recovery center -- where she checked in on Aug. 29 for "stress and anxiety" -- to perform at the Eau Claire Regional Arts Center in Wisconsin.

Rimes played a full 70 minute set despite her recent breakdown -- which stemmed from a public battle with a group on Twitter ... who kept blasting Rimes for her relationship with Eddie Cibrian.