off one's game

(idiomatic, by extension) Performing in any activity below one's usual level; behaving in an irregular, inept, or awkward manner; feeling unwell.

Example: c. 1910, Ralph Henry Barbour, "The Dub" in The New Boy at Hilltop and Other Stories:
  One fellow I knew once was off his game the whole first half because some idiot was flying a kite over the field advertising some one's pills.
2008 April 2, Tracy Connor, "'Mellow' shark dies," Daily News (New York) (retrieved 9 Sep. 2008):
  Big Bertha, the crowd-pleasing sand tiger shark who survived an astonishing four decades at the New York Aquarium, has died. "She was off her game. She slowed down. She wasn't quite right," said Hans Walters, the animal department supervisor.