I had a similar experience during my current 40 day fast. I am on day 27.
I have been drinking distilled water since the beginning of my fast until the 24th day. I noticed that my body was weak and I didn't have any energy, but it was to the point where I felt like I would be malnutritioned.
From 20-24 I began vomiting brown vile. I'll just keep the details to myself. It was pretty ugly.
My energy level started to swoop down pretty low. I felt as if my body was telling me, your missing out on simple elements needed for the body to function.
Then I switched to mineral water. I don't know what your opinion on mineral water during a fast is and certainly I don't care. My body is still in ketosis as my lungs still smell like acetone and I feel weakness, but not the type of headache associated weakness I had during the time I drank distilled water.
IMHO, I think mineral water is great for water fasting.