The model, Xristina Marie, reportedly told the UK Sun they "fooled around" weeks before the former couple's split was revealed.
Smart is denying the Sun's story, telling E! News, "There is no truth to any meeting or physical encounter. I am weighing my options on the appropriate action to take against Xristina Marie and/or The Sun."
J.Lo and Smart called it quits after three and a half years of dating, E! News confirmed Friday. "They were hoping to be more low key about things," a source said. "He was slowly going to move out and that would be it."
The duo first met not long after the "I Luh Ya Papi" crooner and her husband Marc Anthony split up in 2011, with Smart soon appearing with J.Lo on the red carpet.
Although sources said J.Lo and Smart's breakup was "a long time coming," the two are still on good terms.
"She will definitely still keep things civil and actually still loves him," an insider told us. "The hard part is the kids love him too. He has become a part of her family."
"It's not going to be one of those breakups where you never see them together again."
Another source said the triple threat star was "so embarrassed" when the sexting allegations came out, adding that J.Lo believed Smart when he vehemently denied he was hiding some sort sexting relationship he had with a transsexual, who tabloids claim he met via Instagram.
"He's never cheated on her," the source said.