"She looked phenomenal," gushed concertgoer Scott Pierce, 27, a local internet technology sales rep.
But Simpson – who seemed nervous and tentative at times – told the crowd of 15,000 that it had been "a rough week."
At one point in the show she said, "You definitely have to confront everything you go through."
Though the singer thanked fans last week for their support, the stress of the recent scrutiny may have also taken a physical toll on Simpson, who frequently sipped from a mug during the 40-minute set.
"My voice is weak tonight and I feel so vulnerable onstage," she told the crowd, according to the The Grand Rapids Press.
Simpson, who was the opening act for the country trio Rascal Flatts, stopped singing during one of her songs and asked to start it over.
Responding to reports of a meltdown, her rep tells PEOPLE, "Jessica just had an off night. She’s a perfectionist and wanted to start some of the songs over. She always wants to give her best performance to her fans."
Fans Support SimpsonThe audience didn't seem to mind. Shouts of "We love you, Jessica," followed her as she finished her performance and exited the stage.
"She looked very nervous," said fan Tanja Barunovic, 22. "She just seemed very shy, but I think she picked a good selection of songs. I loved her CD. It's fun and girly."
Another fan, Ellen Simons, 16, of Grand Rapids, said, "She looked very healthy. I think she is more of a healthy skinny now, rather than skinny-skinny. Celebrities, if they gain five pounds, they are under the worst pressure. It's just not fair to them."
Despite the minor bumps in the show, Simpson told the crowd she had something to look forward to: a weekend rendezvous with Dallas Cowboys quarterback Tony Romo.
"I'm excited – I get to see my boyfriend tomorrow," she told the crowd, grinning. "I love him so, so very much."
With reporting by K.C. BAKER
Jessica says Tony would never tell her to lose weight