Tensions on the set of “CNN This Morning” have recently boiled over, with Don Lemon allegedly “screaming” at co-host Kaitlan Collins off-camera during an ugly December incident that has left the cast and crew rattled ever since, The Post has learned. After the party, Lemon had a late-night dinner with his agent, Jay Sures, according to sources. While he dined with Lemon, Collins met with the show’s other co-anchor, Poppy Harlow, the sources said. “It’s messy.”Co-hosts Poppy Harlow (left) and Kaitlan Collins (right) arrived at the White House Christmas party without Don Lemon (center) after his blowup earlier that day. CNN CEO Chris Licht was upset by Lemon’s behavior toward Collins and had the anchor take the next day off, sources said.