A Rihanna fan is claiming that the sexy singer gave her herpes -- but not in a way you'd brag about.

Starkeema Greenridge of Harlem did not contract the herp from swapping spit or making out with Rih. Instead, she believes she got it by sampling a tainted tube of RiRi Woo, the singer's red lipstick from her brand new MAC collection.

Greenridge, 27, tested the product on May 7 at Rihanna's concert at the Barclays Center in Brooklyn -- after other concertgoers also tested it. A-ha. So she actually got herpes from a fellow Rihanna fan ... or so she thinks.

"I asked [the M.A.C. employee] if it was clean, and he said, 'Yeah, yeah, yeah,'" Greenridge said. "Two days later, my upper lip was swollen with blisters.”

Ick. Cosmetics testers are breeding grounds for bacteria, especially outside of department store settings or retail counters, where there are bottles of sanitizer and alcohol that allow makeup mavens to sample products in a cleanly manner.

No one forced Greenridge to apply the product, and it'd seem near impossible to locate the source of the contagion, but that didn't stop her from getting a lawyer and filing suit. 'MERICA!

Greenridge's attorney, David Jaroslawicz, said his client was a big RiRi fan pre-herpes. But these days? Not so much. "Now I guess she’ll listen to Chris Brown," he said.

Yeah, he went there. You stay classy, Dave.