Not a doctor, but with a bit of research on the subject found this is probably a very MINOR case of refeeding syndrome which typically occurs after extended periods of fasting / starvation (7+ days). Since this your fast is a smaller scale, I’m guessing it’s minor with less severe side effects (i.e. diarrhea). You can read up on it on PubMed: www.ncbi.nlm.nih.gov/pmc/?term=Refeeding+syndrome
From what I gathered, the best solution is to ensure sufficient electrolytes are taken during and after fast (I recommend SaltStick tablets), proper vitamins are taken (specifically Vitamin B), and slow refeeding, 50% of daily intake w/ easily digestible foods.
It could also just be your body and is fairly common among fasting practitioners. It will pass, just stay close to the bathroom.