Well done on reaching Day 21. Excellent! Weight gain is not uncommon during a fast. If you're only consuming water, then any weight gain has to be water retention offset by any body reserves consumed during the day. As you mention in another post, the pending period is likely to be a factor. In your reduced-toxic state, you might think your period never started because much of the symptoms (pain and bleeding) are associated with detoxification and preparing the womb for childrearing. Having already detoxed for 21 days, you might find reduced or an absence of normal symptoms but by no means be fooled into thinking that the rest of the process was omitted.
Water fasting is a health-building process and weight loss is merely a coincident side-effect of the fast. Don't worry about not seeing a steady loss every day as the overall loss cannot help but continue.