CHICAGO -- Roy Halladay has pitched in hot, humid conditions before, but this time it ruined him.

"I've never had something like that," he said.

He left Monday's 6-1 loss to the Chicago Cubs at Wrigley Field in the fifth inning because of heat exhaustion. The heat index Monday reportedly hit 103 degrees. But Halladay said Tuesday he felt better and expects to make his next start Sunday against the San Diego Padres at Citizens Bank Park.

He said he still can't explain why the heat affected him so much. He said he consumed plenty of fluids, a process he said begins the day before he pitches.

"It seemed like in between innings, I couldn't get away from it," he said. "I think it just got to the point where it just kept continuing and kept getting hotter and hotter, and I couldn't stop it. It was a combination of everything -- heat, travel -- I think it got me at one point. In that last inning, there was a point where I felt like I was about to get wheeled off the mound."

Halladay declined to say if he received an IV but said two Cubs doctors treated him after the game. He said he cut back his post-start workout Tuesday, but said it should not affect him with an extra day to prepare because of a day off Thursday.

"Tomorrow hopefully will be more of a regular day," he said. "The big thing they were concerned about was the cardio today. It was just making sure I didn't get that high rate up and everything again."

Oswalt throws first bullpen session since injury

CHICAGO -- The fourth member of the highly touted Phillies rotation could be back in the near future.

Roy Oswalt threw a 48-pitch bullpen session Tuesday at Wrigley Field, which was the first time he has thrown off a mound since he went on the disabled list last month with lower back inflammation. Oswalt said he is pain free -- the back pain had extended into his left leg -- following a cortisone injection on July 8. He said he received a cortisone injection following his start in Arizona in April but said that shot never worked.

"It felt great," he said. "The big thing now is building arm strength and trying to get my other pitches again."

Oswalt said he plans to throw another bullpen session Friday. If that goes well he could begin a rehab assignment next week.

Worth noting

• The Phillies said third baseman Placido Polanco's injured back improved Tuesday. He requested a cortisone injection Monday to relieve the inflammation, but the team said no injection has been scheduled.

• Triple-A Lehigh Valley relief pitcher Jason Grilli has an out-clause in his contract if another big league team offers him a spot on the 25-man roster. The Pirates are close. The Phillies must release him unless they promote him. Grilli is 4-1 with a 1.93 ERA, but there are indications they are not prepared to promote him.