Ruined Metabolism!

I haven't posted for a while but I have a very serious concern if any of you have experienced this. I have been an on and off faster for about two years. I started fasting about 3 times per year. The first water fast, I weighed about 120, which I had never been heavier, at 5'2", VERY active runner, and lost about 10 pounds in 5 days and kept it off for about a year, gained about 3 pounds later. Then I started gaining weight when I moved and life was stressful and I fasted again for 5 days and lost about 8 pounds, but I was heavier at about 127. I've been trying to lose weight and keep trying to fast for 5 or more days and every time I lose less weight, like 1/2 pound every two days, to the point where I give up because I am not losing anything. Now I weigh 135, am running more than I ever have, and eat less than I ever have. If I eat anything, I gain 1 pound that day and it doesn't come off. I've kept detailed journals and one day I ate about 1,200 calories and two days later, my skin was painful from stretching and I weighed 2 pounds more and it stayed on. My thyroid is fine. I really feel as though my body is not burning any calories to maintain itself. The last time I fasted for 7 days on water, I only lost about 3 pounds and I weighed more and ran more. I have tried eating healthy and juices for months and with that, I simply maintain my weight and if I cheat for a day, I gain a pound that stays on. So, is my body permanently ruined? Does this mean that I literally have no choice but to fast for severe lengths of time in order to lose weight? Has this happened to anyone? I used to love fasting!