If this is a water fast for weightloss when you only weigh 55-56kgs (121.lbs) I hope you have a doctor on board. You could be at risk of heart atrophy, which will kill you and heart athrophy isn't something that happens with small symptoms, most people don't notice it happening and they just drop dead. It's basically when your body starts eating muscle and may eat the heart muscle and you die. This isn't something that happens everytime a person water fasts but it tends to happen when people don't have much weight and try to lose weight this way. Another thing that can happen is you could have a heart attack if you decided to stop the fast suddenly and start eating or binging. You will need to refeed slowly.
In heavier people the chance isn't as risky unless they have heart problems to start off with, but you're already at a low weight which is why I'm asking. I'm really big and I'm only doing 30day water fasts with a few weeks break inbetween and I have a GP looking at my heart every week as well as checking my bloods. I also was a weight lifter for some time and i have a lot of muscle mass.
Water fasting is definitely an amazing experince, but it's not a quick fix and if done incorrectly it can go terribly wrong. Please seek a GP, just at least to do an EKG before you attempt this if anything to make sure you have nothing going on with your heart already =) All the best with your journey.