I have had the same thing happen -- lost weight then gained some back. I believe that I was retaining water because of processed foods with a lot of salt. Have you added any bacon, sausage, lunch meats, pepperoni, cheese, canned tuna or veggies, etc. or any other food that has a lot of salt? Also, have your measurements changed? Could you be gaining muscle that weighs more than fat but takes up less space? For a long period of time I did not lose any weight, but I did lose a lot of inches because I was gaining muscle.

If none of the above applies, just stick with it -- count the carbs, eat healthy, exercise, and your body will come around.