Cubs manager Rick Renteria said they noticed Jackson shaking his right hand during the inning.
"He started shaking his hand a little bit," Renteria said. "The best thing to do was get him out of there."
Jackson threw 87 pitches, giving up seven hits, including Rene Rivera's home run in the fifth.
The Cubs will know more about Jackson's status on Friday. He said he's had cramping in his hand before, and did not miss his next start.
"It's one of those things where something can go wrong, it does," Jackson said.
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.