NEW YORK (AP) -- Lady Gaga says she's "heartsick" to postpone four shows after sustaining an injury that's left her unable to walk.

A news release says that, in addition to Wednesday's concert in Chicago, performances set for tonight, also in Chicago, Saturday in Detroit and Sunday in Hamilton, Ontario, Canada, have been postponed due to a case of synovitis, a severe joint inflammation.

Gaga took to Twitter to explain, saying she injured herself during a performance some time ago and her condition has worsened, leaving her immobile following Monday's concert in Montreal. She's been hiding the injury from her staff, but can no longer perform.

"I've been hiding a show injury and chronic pain for sometime now, over the last month it has worsened," she wrote.

The remainder of the "Born This Way Ball" tour is expected to continue on schedule, beginning with a two-night stand in Philadelphia on Feb. 19-20.