Jackson said he had trouble catching his breath after chasing a foul pop down the right-field line. He has been feeling poor for the past few days and spent Tuesday's off-day in bed trying to get as much rest as possible.
"I just didn't want to come out of the lineup," Jackson said.
Chest X-rays were taken at the hospital, and he was given some antibiotics. Jackson, who had returned to the clubhouse by game's end, said the doctors told him it was possible he had pneumonia.
"They're going to start me off on antibiotics and see what happens," he said. "It's just unfortunate. You don't want to start the season off with the flu or something that keeps you out of the lineup."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.