The 32-year-old singer was onstage at the Hollywood Casino Amphitheatre in Tinley Park, Illinois, on Friday (29Jul16) when she began to belt out her 2012 single, Over You.
During the performance, Lambert became visibly emotional and could barely get the words out, and had to rely on her fans to help finish the emotional track, Us Weekly reports.
The singer allegedly told the audience she was having a rough night, as she was feeling hormonal and had been suffering from cramps.
Later on in the set, she broke down once again while performing The House That Built Me.
"She was amazing," one concertgoer told Us Weekly. "But emotionally unstable. Crying the whole show."
Fans took to social media after the concert to share their concern for Miranda, with one Twitter user writing, "@mirandalambert made me cry from her crying."
Lambert and her ex wrote Over You as a tribute to 40-year-old Shelton's late older brother, who died when the singer was a teenager. The heartbreaking song won the couple the 2012 Country Music Award for Song of The Year.
The stars stunned fans last summer (15) when they announced they had divorced after almost four years of marriage.
Shelton has since found love again with his fellow The Voice coach Gwen Stefani, while Lambert has moved on with country singer Anderson East.
However, she recently reflected on her tough year in a candid post on Instagram.
"The last year of my life has been one of heartache and healing, ... Of learning to be honest .... Accepting the flaws and celebrating the smiles. Finding peace in dark places ... having some moments alone with me," she wrote. "Facing fears and feelings... All of them."