If you have a weak stomach when it comes to illicit tales of age-inappropriate sex and the alleged two-timing of a teenage mom, then you might want to chug some Pepto before reading the new In Touch cover story on Jamie Lynn Spears and her fiancé, Casey Aldridge.

Jamie Lynn introduces Maddie to Wal-Mart on Aug. 6.

"Jamie Lynn betrayed," screams the headline. "Casey cheated on her!"

In an "exclusive" interview with the mag, a 28-year-old class act named Kelli Dawson details her alleged yearlong affair with the 19-year-old Louisiana pipe layer (no, that's not a euphemism), who is reportedly livid over the accusations.

"He totally instigated it and pursued me," Dawson tells the mag of Aldridge, whom she says she met in late 2006 (for those doing the math, that would mean she was in her mid-20s and he apparently wasn't yet old enough to vote). "His friends would tell me that he liked me, that he thought I was pretty."

Dawson, who poses for the magazine in cowboy boots, a butt-skimming minidress and a wooden cross, says they became "hot and heavy" in the spring of 2007, despite his continued involvement with Jamie Lynn, 17.

"You never knew if they were broken up or not," she explains, "because it changed from day to day!"

Still, she says they kept their distance -- or at least certain parts of them did -- when Jamie Lynn was six months pregnant with daughter Maddie, who arrived in June.

"It just didn't feel right," she explains of purportedly nixing the nookie in March. "He had to deal with the baby coming, and it had to stop." Sort of. Kelli maintains they'd still "kiss occasionally."

But Aldridge insiders are taking issue with Dawson's account, telling TMZ that he's "ballistic" over the philandering claims and hinting that she may simply be "trying to make a buck."

They add that Casey has called Jamie Lynn's family and given his side of the story, which they've accepted.

And what of that photo in the mag showing Casey goofing around with Dawson? It was supposedly snapped before he began dating Jamie Lynn, who in recent days has been photographed out and about sans her husband-to-be.

Dawson, for her part, says she still sees Aldridge "all the time, at least once a week." Anything else? "I still love him and I think he loves me."

And while neither Spears nor Aldridge has officially commented on the allegations, someone we never thought we'd hear from again did.

"Kelli and Casey have been a couple on and off for a few years," says Jason Alexander, who took time out from his busy schedule of telling people, "Yeah, I was that guy married to Britney Spears for 55 hours," to offer his incisive observations to In Touch. "They were a really tight couple, but I think it was hard for Kelli, having Jamie Lynn in the picture."