Sounds like refeeding syndrome. You’ve basically been fasting the past week, and very low calories leads to low insulin leads to reduced water/electrolyte retention. It also leads to increased free fatty acids in the blood, which suppress appetite. Exercise tends to exacerbate this last thing.

Adding carbs back in increases water retention, and along with a gallon of water a day, you’re basically trying to give yourself hyponatremia. This would explain the nausea, vomiting, and general feeling of ickyness.

The usual treatment is to ease back into your regular diet. And salty foods help a lot. I’d suggest some full-sodium broth or stock of some kind. Or Gatorade. Or just a teaspoon of table salt in a liter of water. You should start to feel better almost immediately. If you don’t, see a doctor.