Umm what is your current bodyfat..5%? Can you see your ab muscles. Son is your height ,14 and weighs 144 and is like 10%.
At your weight wouldn't be doing any longer than 7 to maybe 14 max on water.You could drop like 25 pounds that sounds like serious danger zone starvation to me. I would think 7 is good.
And if you can't gain wegiht after that simply up calories. Its simple enought take in more than you burn up and you gain ,cut down and you lose. You can drink lots of calories real real easy. Juice,smoothies whatever. Eat more carbs..potatoes,rice,pastas.And cut down on activity if you are doing a lot for awhile.Eat a lot more nuts they are very high in calories. And some fruits are up there think bananas, avocados. I would sell my soul to the guy downstairs to have your problem!