SAN FRANCISCO — Shawn Kelley didn’t suspect anything was wrong with his right elbow when he jogged out of the visitors’ bullpen and finished tossing his warmup pitches in the sixth inning Monday night at AT&T Park. Then, in his first pitch in a game in a week, the Washington Nationals right-hander felt a familiar sensation as he watched Mac Williamson smash a 464-foot, two-run home run.

Right away, Kelley said, he felt something wasn’t right. He wasn’t getting the proper extension. The ball didn’t have life. His next two pitches were an 89-mph fastball for a ball and a 78-mph slider for a strike. He then spiked a pitch, which coaxed trainer Paul Lessard out of the dugout. That ended Kelley’s night. After the game, Nationals Manager Dave Martinez said Kelley is dealing with ulnar nerve irritation, a troubling diagnosis for a pitcher who has undergone two Tommy John surgeries. He is likely headed to the disabled list.

“It’s another day in the life of my elbow,” Kelley said.

Kelley hadn’t appeared in a game since throwing six pitches April 16 against the Mets. But he got up in the bullpen to warm up at least a couple of times during the idle period and maintained he felt better than he has in years, which he said made what happened Monday unexpected.

“I know it’s early, but for me, for April, especially in the cool weather, my velocity’s been up, my strength’s been good,” Kelley said. “The stuff I’ve been doing in the weight room and training room with the guys has been really good. It’s probably been the best I’ve felt in years …

“This one is kind of puzzling. Like I said, I can sometimes point to something or think back to something, but it was kind of a head-scratcher this time.”

If Kelley goes on the disabled list, it’ll be his fourth stint in the past 11 months. He spent time there last season for back, neck and elbow injuries. He took an injection for the elbow last fall to quicken recovery, ending a season in which he posted a 7.27 ERA in 26 innings and the highest home run rate among relievers in baseball.

“You know, I’ve been through so much crap in my arm and my career that I just try not to get too concerned or think too far down the road,” Kelley said. “Come in tomorrow and see. Sometimes it bounces back. Get some rest, come back tomorrow and do some exercises, do some stuff, and get with the trainers and doctors and see where it’s at. Usually I can tell in a couple days if it’s starting to calm down or it’s something more serious. I’m just going to take a few days to rest and get back to throwing a baseball.”

Kelley, 33, signed a three-year deal worth $15 million before the 2016 season. He began his Nationals tenure by pitching to a 2.64 ERA and striking out 12.4 batters per nine innings in 67 appearances. The performance as Washington’s primary setup man put him in competition for the closer role last spring training, and he was given the job in April before landing on the disabled list the first time. Now he has allowed 15 home runs in 32 innings the past two seasons, including three home runs to the past 15 batters he has faced, and his immediate future is unclear once again.

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