"It started very much about work. He was completely thrilled. By the time he was leaving for tour, it had become a lot flirtier," says a source

Miranda Lambert‘s new boyfriend Evan Felker blindsided his estranged wife Staci Nelson with his sudden divorce filing in mid-February after his relationship with the country singer turned romantic.

“The notion that [Felker and Nelson’s] divorce happened for any reason other than that is complete bulls—,” a source close to the couple exclusively tells PEOPLE of Lambert and Felker’s relationship, news of which broke last Tuesday.

According to the source, Lambert and Felker, both 34, connected via text message before his band Turnpike Troubadours opened for the country superstar during her Livin’ Like Hippies Tour for three dates in early February.

“They had never met and she had started texting him about looking forward to playing together and getting to know each other and maybe they could write because she really liked his writing sort of thing,” says the source. “It started very much about work. He was completely thrilled. By the time he was leaving for tour, it had become a lot flirtier.”

Miranda Lambert and Evan Felker
| Credit: Jason Merritt/Getty; Frazer Harrison/Getty

Reps for Felker and Lambert did not respond to PEOPLE’s requests for comment.

During their one-and-a-half-year marriage, Felker and his wife Nelson, 33, were like “best friends,” says the source, and he was open with her about Lambert’s text messages before heading out on the road.

“He knew it was flirty and crossed a line with Staci. He showed her the whole thing and felt bad about it. It wasn’t sexual or anything, but he was like, ‘I don’t want to blow her off because this is the kind of money that can change our grandkids’ lives if I’m able to write with this person,'” adds the source.

Then the tour came, sending Lambert and Felker on the road together for three dates from Feb. 1-3.

“[People are] making it seem like, ‘Oh, they got close while they were on tour together,’ as if the tour was six months long, but it was three days, and the first two of those he was still acting like he was in love with his wife,” the insider says, adding that Felker “didn’t even call home the third night.”

RELATED: Miranda Lambert Dating Turnpike Troubadours Singer Evan Felker After Anderson East Split: Report

When the tour ended, Felker allegedly skipped his flight home to Oklahoma — where Nelson and Felker lived as newlyweds — and continued traveling with Lambert, telling his wife via email, “I’m not coming home this week,” according to the source.

Felker then “full-on ghosted” Nelson as Lambert shared cryptic posts via Instagram, the source says. On Feb. 7, Lambert posted a photo of her smiling in front of a mirror featuring the words “I do this thing called whatever I want” written in red.

And on March 7, Lambert posted a photo of graphing paper, which the source says Felker uses to write lyrics, and Modelo, his favorite beer. “Tools. #Texas #cowrite #happyhours,” Lambert wrote in the caption.

The source tells PEOPLE that if it wasn’t for Felker’s relationship with Lambert, “he would be talking to” his estranged wife and wouldn’t have filed for divorce on Feb. 16, adding that “the notion that he was planning to divorce [Felker] before he met her is complete insanity.”

Credit: Staci Nelson/Instagram

But Felker eventually called his wife in late March and “laid it all on the table, like, ‘Surprise!'” the source tells PEOPLE, adding, “He’s always the kind of guy where if he screws up he comes clean.”

“He got sucked into it. He’s not a victim; he’s a grown man and should have said no,” says the source of Felker and Lambert, whose split from ex Anderson East was revealed in April following weeks of breakup speculation.

In the midst of the drama, Nelson alluded to the cheating allegations in a “public service announcement” posted to her Instagram Story on Thursday. “PSA: If Staci Felker can make it through this week, so can you,” Nelson, who continues to use Evan’s last name on social media, captioned a teary-eyed picture of herself.

Credit: Staci Felker/Instagram

One day after news broke that Lambert and Felker were seeing each other, Lambert’s former husband Blake Shelton seemingly called out his ex on Twitter.

“Been taking the high road for a long time.. I almost gave up,” Shelton, 41, wrote on social media Wednesday morning. “But I can finally see something on the horizon up there!! Wait!! Could it be?! Yep!! It’s karma!!”