i don’t think you should trust any online source rn, but the most likely causes are UTI or medication related
u should get medical attention at a GP asap, and probably abstain from fasting for a while until the problem gets resolved
It’s bile overflown to blood from the biliary system of the liver-gallbladder.
Your gallbladder should empty itself when you eat food but when you fast it cannot excrete the yellow coloured bile into the stomach. There’s lots of bile after 8 days of fasting that overflows into the blood serum.
This is why prolonged fasting may promote gallstones / sludge formation as the bile that contains calcium, cholesterol and other substances could get condensed in the gallbladder. Avoid this by staying hydrated at all times.
Best way to get the GB burst out the bile is olive oil, that of course will break your fast very unpleasantly.