Schumaker grabbed at his left hamstring after hitting a fly ball to Hunter Pence in right field for the second out of the bottom of the fifth inning.
"It was a weird thing," Schumaker said. "Maybe too much caffeine, too much excitement -- I don't know what was going on. Every time I swung or sat down, it would lock up.
"It's not a pull or anything. It's just a cramp. I've had it before."
Schumaker was replaced at second base by Ryan Theriot in the top of the sixth inning. Theriot had four hits in the Cards' 3-2 loss to the Phillies in Game 3 on Tuesday.
The apparent injury to Schumaker wasn't the only odd twist to his at-bats, as play was halted briefly after a squirrel crossed in front of home plate earlier in the at-bat.
"I'm glad I got taken out of the game," Schumaker joked. "I don't know what was going to happen next."
Schumaker had two singles and scored a run before leaving the game. St. Louis went on to win, 5-3, to force Game 5 on Friday in Philadelphia.