FILE - This Dec. 15, 2012 file photo shows singer Lady Gaga performing at the Prudential Center in Newark, N.J. Lady Gaga's tour confirmed Tuesday, Feb. 12, 2013, that the singer has postponed performances in Chicago, Detroit and Canada from Feb. 13 - Feb. 17, due to a severe inflammation of joints. 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 Tuesday news release says performances set for Feb. 13-14 in Chicago, Feb. 16 in Detroit and Feb. 17 in Hamilton, Ontario, Canada, have been postponed. 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. I've been praying it would heal. Neither Lady Gaga nor the news release specifies what the injury is. 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. Makeup dates for the missed shows will be announced later.