“Is Kate Nash mentally stable?” I found myself asking at Nash’s sold-out show at Lincoln Hall on Monday night. As the show went on, it became clear the answer was a clear and resounding “no.” Nash, the once-sweet and adorable British singer, has lost her marbles. Nash walked onstage with crutches and played with one leg propped up, due to an injury she got when she was “wrestling somebody, but not in that way.” That was the first clue I realized Nash might not be all there anymore.

Touring to promote her brand-new album, My Best Friend is You, Nash showed a darker side Monday that strayed far from her cutesy days of “Merry Happy.”

Opening for Nash was Supercute!, a band of three 15-year-olds who sang about pigeons and candy, while the crowd wondered what they were doing on stage. Wasn’t it past their curfews? The band wore plastic, glittery outfits while playing the guitar, keyboard and ukelele, even performing one song while hula-hooping. Thankfully, Supercute! only played a few songs, because they were, simply put, super weird.

Nash and her four-man band came on about an hour later. The first half of the set was familiar enough, and reminded fans of her earlier work. Even new songs like “Do Wah Doo” and “Paris” showed an inkling of the Nash that used to be­-sweet, clever, girly but brutally honest and a bit profane.

It wasn’t until the show was well into its second half that serious concern for Nash’s mental health set in. Between throwing back some Goose Island and dropping the f-bomb every other word, Nash introduced a scary, new “screamo” musical identity with tunes like “Mansion Song,” which begins with an angry bout of profane slam poetry. “I take cocaine/I don’t give a f*** about her, I want your name,” she screamed.

The concert wrapped up when an audience member crowd-surfed his way to the stage upon Nash’s request. She then had the young man, no older than 14, sit at the keyboard while the band played around him and Nash screamed angry lyrics.

Nash finally left the stage. And with her exit, she left the audience wondering what will become of her in the next few years, and if we’ll ever hear a return to the sweet and innocently edgy days of “Made of Bricks.”