Ortiz, who had been examined for similar problems earlier in August, was pulled from the lineup just before Monday's game because of heart palpitations.
There was no immediate indication when Ortiz, who leads the American League with 47 home runs, might be able to return to the lineup.
The Red Sox on Thursday said rookie pitcher Jon Lester was also undergoing tests by team medical personnel. Team officials confirmed that Lester, 22, was found to have enlarged lymph nodes. The pitcher was placed on the disabled list Monday, with a strained back being listed as the cause.
Lester has a 7-2 record and a 4.76 ERA in 15 starts with the Red Sox after being promoted from Triple-A Pawtucket on June 10.