Entering Friday's action, Pena was hitting .209 with two home runs and 10 RBIs.
Washington manager Manny Acta declined to go into specifics, but he said Pena will have his shoulder looked at during the All-Star break. Pena declined to comment, saying there was nothing wrong.
In an interview with MLB.com a couple weeks ago, Pena said health was not an issue, but he was stumped when it came to determining what was wrong with his swing.
"I wish I knew so I could find out," Pena said. "That's why we are working hard and watching a lot of video. That's what we are trying to find out, especially me. I'm trying to do the best that I can. It's not over. We have a lot of games to play."
Meanwhile, Ryan Zimmerman took live batting practice on Friday for the fourth straight day. This time he was given a steady diet of breaking balls, and he didn't have any problems with his left shoulder. Zimmerman, who most likely will start a rehab assignment next week, said the shoulder is feeling stronger.
Zimmerman hurt the shoulder sliding head-first in a game against the Orioles on May 18.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.