NEW YORK (February 13, 2013)--Lady Gaga says she's "heartsick" that she’s had to postpone four shows after sustaining an injury that left her unable to walk.

A new release says her performances on Feb. 13 and Feb. 14 in Chicago, Feb. 16 in Detroit and Feb. 17 in Hamilton, Ontario, Canada, have been postponed because of a case of synovitis, a severe joint inflammation.

Gaga tweeted that she injured herself during a performance some time ago and her condition has worsened, leaving her immobile after Monday's concert in Montreal.

She said 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".

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 and Feb. 20.

Makeup dates for the missed shows will be announced later.