It depends on what you mean by "reject" and what kind of carbs you ate? If it was wheat based and you haven't had any wheat in a month and you "rejected" it by getting sick then there's a good chance that you just discovered that you are wheat/gluten intolerant. A lot of people discover that intolerance after eliminating grains and then cheat.

Honestly...I don't know if having a cheat day is the best idea. With every other diet I have tried in the past, I always allowed myself a cheat meal or cheat day and found that it was very easy to get off track that way.

With switching to the keto lifestyle, I have found that not cheating is a lot easier because I don't want to bump myself out of ketosis, not to mention your cravings eventually disappear and are much easier to handle when you completely eliminate foods with super high carbs.