"I barely know what to say," she writes on her Facebook page, Tuesday. "I've been hiding a show injury and chronic pain for some time now, [and] over the past month it has worsened. I've been praying it would heal. I hid it from my staff. I didn't want to disappoint my amazing fans. However, after last night's performance I could not walk and still can't." The pop star, 26 – whose Twitter page explains that she has a case of synovitis, which is severe inflammation of the joints – was forced to postpone two concerts in Chicago, one in Detroit and one in Hamilton, Ontario. "I hope you can forgive me, as it is nearly impossible for me to forgive myself," the rest of her Facebook post says. "I'm devastated & sad. It will hopefully heal as soon as possible. I hate this. I hate this so much. I love you and I'm sorry."