No, fat breaking down isn’t considered a “high fat diet” because you’re not eating the fat, your body is breaking it down.
My guess is that you’re losing weight too fast. 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.