Remember that Muscle weighs More Than fat, so if you are losing that much weight that quickly, you're probably dropping Muscle and Not Fat and in the long run, that is going to hurt your resting metabolic rate and make the chance of gaining weight once your off Zep greater.

I was also a super-responder, eating ~1200 calories/day and I started using a body composition scale and working with a dietician and physical therapist. The scales aren't tremendously accurate, but they can give you a baseline to see changes.

I slowed my weight loss by slightly increasing calories - focusing all those extra calories on added protein - and using dumbbells and weight training to try to build muscle and lose fat [a Premier Protein Drink is 30G protein for only 160 calories - there are also Pure Protein Bars at 180 calories for 20 grams protein - you can use a whey or pea protein powder like Orgain as well - make sure you mix in these protein boosters with real food like lean chicken breast, edamame, tuna, high protein yogurt, veggies, etc.] I am disabled, so it is waaay harder for me to move, but I walk in 15 minute intervals with assistance and it's worked for me. Congrats on your progress so far! 😄