Why is it I am dieting and gaining weight?

I have been dieting for over a year now and lost 155 pounds. I have also been going to the gym 3 times a week and on my off days from the gym been walking. I am on a low carb diet and it was all working fine until the last couple of months where I would loose a few pounds then gain it back the next week. I would start my diet from the beginning where you would go 2 weeks and eat only 20 carbs a day, then after the 2 weeks you could eat between 40 and 60 carbs a day. IT would work where I would lose weight during those 2 weeks then after the first 2 weeks I would gain it back. Haven't gone off my diet or changed anything. Also lately I am hungry all the time. Does anyone have any ideas on what I can do to make my weight loss start again. I want to lose 35 more pounds by Thanksgiving.