One big reason is an increase in exercise and muscle mass. Muscle weighs more than fat, but is leaner. So, while adding muscle can make you gain weight, your appearance will still be thinner. Don't be too dependent on the scale. Always be sure to assess how you feel and how your clothes fit as your primary methods of determining your success.
Some plateaus can last for months.
Though it's difficult, the best thing you can do is try not to be discouraged, and remember that eventually all plateaus will break. So hang in there, and be proud of all the progress you've made to get to where you are. You've been hugely successful. The best ways to stave off a weight loss plateau generally involve boosting your metabolism, not decreasing your calories. Reassess your exercise time and intensity.