"I felt some sharp pain," Castillo said. "I don't know what happened. I was running hard and tried to beat the throw and felt some sharp pain."
He will be examined on Friday in Chicago.
"I want to finish the season hard," Castillo said. "I feel my body this year is in really good shape, good health. Stuff happens, and you go and give 100 percent, and that kind of stuff can happen."
Was he a little worried?
"Yeah, because I want to finish strong," Castillo said. "The doctor will decide what to do [Friday]."
J.C. Boscan replaced Castillo in the lineup in the bottom of the inning.
Castillo, in his first season as the Cubs regular catcher, has done well lately, going 9-for-29 on the 11-game road trip, which ended Thursday, with four home runs, a double and seven RBIs.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.