Things have gone from bad to worse in Kevin Costner and Christine Baumgartner’s contentious divorce. More than a month after she filed to end things, a new report emerged suggesting the handbag designer, 49, may have gotten too close to tech whiz Daniel Starr during her 18-year marriage to the Yellowstone star, 68. Starr, 42, who’d rented out the guesthouse on the estranged couple’s $145M Carpinteria, Calif., estate, denies the claim. “Kevin had suspicions that Christine could’ve cheated,” confirms a source. “He was often away filming for long periods and she’d refuse to visit, which only made him grow increasingly suspicious.” Meanwhile, she’s criticized Kevin for informing kids Cayden, 16, Hayes, 14, and Grace, 13, of their divorce over a 10-minute Zoom call without her present. Christine is also asking…