The 38-year-old, who won the 2010 National League MVP, revealed the issue had been lingering for a while, dating back to the 2015 season.
Votto will have surgery on Friday, which will repair his torn left rotator cuff.
"I don't know how to explain what exactly happened, but it's been lingering for a while," he said.
"I've had a difficult time with it. Only the past maybe three months or so has it been painful to the point where it's difficult to lift [and] hurts to sleep.
"I'm not sure it's affected my performance. … I can't say that. I don't want to make an excuse."
Votto has batted .205 with 11 homers and 41 RBIs across 91 games this season for the Reds who have a 46-70 record.