I am on day 14 of my water fast. Never even felt dizzy and not very weak either.I have maybe heard of fainting when standing up quickly from lying or maybe sitting. They say to get up slowly on fast and breath a few extra times.
But it does sound strange what happened to you.Agree juice fasting is likely safer or no fasting at all.
You could get seriously hurt or hurt others if you pass out like that at critical times.