What Does Unexplained Weight Gain Indicate?

I am a 46 year old female and had lost around 50 lbs. last year. I have been able to maintain the weight loss by eating several small healthy meals/snacks and drinking over a gallon of water each day. I usually don t eat anything after 7pm at night and eat very little pastas, breads or other carbs. I am 5 4 and was down to 123 lbs. but since the beginning of the year, I have gradually been gaining weight, with no change to my diet. I am now up to 144 lbs. and am very concerned with what is happening since I haven t done anything to justify the weight gain. I have an appointment with my OB/GYN next week, and am going to mention this to her, but what could this be?