Commons came off the bench to earn a spot-kick in the second-half but then saw his effort saved by Neil Sullivan.
"The flu hit me on Tuesday - it was like hitting a brick wall. One minute I felt fine and the next I was in bed sweating and really bunged up," he said.
"I have not felt right since but it was such a massive game for us I thought I would go and play a part.
"I slipped before I hit the penalty and scuffed it into the goalkeeper's hands. I'm very disappointed."