My guess is that you’re losing weight
. 80 lbs lost in 4 months (20 lbs/month) is way too much. Safe weight
loss is 1-2 lbs/week or 8-10 lbs/month. If you’re only eating 1 meal/day, but drinking an entire gallon of water, you’re not eating enough, especially, fiber. Your body needs a certain amount of calories each day in order to be healthy. If you aren’t eating enough and exercising, you’re breaking down muscle instead of only losing fat.