Stalls are the most frustrating part of this journey but also a necessary evil so our bodies can adjust. 18lbs is a major loss in this amount of time so try not to be too mad at your body, as it just worked really hard for you. These stalls are going to happen again and again and unfortunately there is no written rule as to how long or why, but take solace in the fact that if you are following your guidelines, your body will do remarkable things for you. Best of luck!