Given the description you've written about "putting air in the colon", the doctor probably performed a colonoscopy. That is an endoscopic procedure used to inspect the bowels. In other words, the doctor inserted air in your anus to see your bowels better. But your bowels were probably not clean enough (contained too much stool) for a proper diagnosis, so he recommended you to take fiber and senna-pills to clean the bowels more. Anal discharge could be due to rectal inflammation (proctitis). You should follow your doctor’s instructions and perform another colonoscopy at your earliest convenience.