"It slowly built and built and when we were on the break it kind of hit a peak," said Castro, who tore his ACL and had surgery in March 2011.
The Astros may need to make a decision on Castro because if he's unable to catch they have no backup for Chris Snyder, who caught all 12 innings on Saturday night and withstood a jarring home-plate collision on Friday.
Castro said he's not sure whether he'll be able to play on Sunday, for the finale of the series against the Giants.
"It's a day-to-day thing," he said. "It felt better at certain points today than the day before. We just have to watch it on a daily basis."