09/13/2008 05:12 am ET | Updated May 25, 2011

Jamie Lynn's Fiance And Baby-Daddy Cheated, His Ex Tells Magazine

For months, rumors have circulated that Casey Aldridge has cheated on Jamie Lynn -- but no one has come forward to confirm them, until now. In an exclusive interview with In Touch, 28-year-old Kelli Dawson reveals that she and Casey, 19, were romantically involved -- and were still sleeping together when Jamie Lynn, now 17, was six months pregnant with his child. No fleeting hookup, Casey and Kelli's relationship was both real and lasting -- and it grew in intensity, even as Jamie Lynn announced she was pregnant. And, Kelli says, they still meet up on a regular basis. "I see him all the time, at least once a week," she shares.

The pair first met in November 2006, at the home of her sister, Kim Toller. At the time, Casey was already dating Jamie Lynn, but the sexual chemistry between Casey and Kelli was instant. "We were both really attracted to each other," Kelli reveals. "He totally instigated it and pursued me. His friends would tell me that he liked me, that he thought I was pretty." The couple saw each other a few times and "kissed randomly," but it wasn't really "hot and heavy," Kelli admits, until the spring of 2007. Though Casey and Jamie Lynn were still involved, Kelli insists, "You never knew if they were broken up or not, because it changed from day to day!"

The pair didn't bother to hide their relationship. "Kelli and Casey have been a couple on and off for a few years," confirms Jason Alexander, Britney Spears' first husband and a native of nearby Kentwood, La. "They were a really tight couple, but I think it was hard for Kelli, having Jamie Lynn in the picture."

Kelli says she and Casey stopped sleeping together a few months before Jamie Lynn gave birth to their daughter, Maddie. "It just didn't feel right," she explains. "He had to deal with the baby coming, and it had to stop." She says the last time the couple had sex together was in March but "even after we stopped having sex, we would still kiss occasionally."

For more details on Casey & Kelli's relationship, check out the new issue of In Touch Weekly