First Time Water Fasting I have questions

Hello, I'm 13 and I'm Water Fasting for the first time in my life. I want to go for 2 weeks because I'm slightly over weight and I want to detox my body, today is my second day and I wanted to eat so bad yesterday because my parents did a BBQ and it looked so good but I bared through it. I want to know is it natural to feel weak? I'm a pretty strong kid, theirs these chairs that I put in a room and bring out when theirs BBQs or parties and today I just couldn't carry them. I have a room upstairs and I could barely walk down the stairs without feeling like my legs were going to contract. I also would like to know, could I switch from Water Fasting to juice fasting after 2 days of Water Fasting and still get into ketosis with the progress I made from my water fast? please and thank you!