The pop he felt in his second inning for Class A Rancho Cucamonga against San Jose came on a curveball, Billingsley said.
"It was just scar tissue breaking, like cracking your neck," he said. "It didn't hurt when it happened, it didn't hurt after it. It didn't feel like pain I had in previous injuries."
But, Billingsley said, he'd never had scar tissue break before while actually pitching, so he was concerned and asked to be taken out of the game. He returned to Dodger Stadium on Sunday night, where Dr. Neal ElAttrache examined him and gave him the thumbs-up.
"I didn't know what it was before I got back here," Billingsley said. "One of our trainers was with me, and he told me it looked OK to him."
Billingsley will take a few days off from throwing before resuming rehab. He's scheduled to make four more Minor League starts.
"I had the adrenaline going [Sunday]," he said. "I was just glad to be out there, to compete. It's great to be back -- well, almost back -- competing."
Earl Bloom is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.