I just listened to a study/podcast and the two doctors who do extended fasts regularly were adamant about breaking the fast if feeling nauseous. It means you’re dehydrated and could be very dangerous. From the rash to all of your other symptoms - you body is trying to signal to you that it needs help.
Edit: you mentioned drinking water but have you been supplementing with salt/electrolytes?
Here are some videos you can watch about fasting. You should watch the one about breaking a fast and what to eat to avoid refeeding syndrome. Protein and Fat heavy for the 1st couple of days eating rich in Phosphorus foods.
WATER FASTING: The Complete Guide (Fastest Fat Loss Method)
WATER FASTING & SALT: Should You Consume Sodium While Fasting? IMPORTANT
HOW TO BREAK A FAST: Best Results & Worst Mistakes (Refeed Syndrome)
Electrolytes? I remember when I was a rookie faster and it was day 17 that hit me like that too. I perked right up after some electrolytes. Always listen to your body though. Underweight says enough, fasting that long probably beyond unnecessary and a lower carb diet would have suffice. After 3 days your immune system would be rebuilt and HGH also boosted.