The reason your doctor may be instructing you to flush is due to your issue of debris buildup, which as you noted can lead to sones. So I presume you may have this issue hence irrigation to keep things flowing smoothly. We are each different and require our individual needs for our specific ailment. Having burning while flushing is odd unless you’ve had a buildup of debris that the saline solution pushed through on the way to your bladder? I’m a retired male nurse and have cared for a large variety of make and female catheter patients in my 40 year career. If the burning occurs each time your flush let your doc know. They may change the irrigation solution, increase the number of times in a day you flush or something else. Hope you find an answer and don’t forget your fluid intake.