It is normal to have a lack of energy while fasting. Make sure to get plenty of rest and sleep. That is important. Yoga and/or meditation are good things to do while fasting. If you have trouble exercising then it is OK to skip the exercise.
See site below for more on fasting including things that can go wrong. Your body is taking energy from your muscles to fix things deep inside of you. It might be good to read something spiritual. Also look at your tongue to see the cleansing going on. That was my best buddy while fasting. It was saying to me "look at all this junk coming out." Also with the internet, you have millions of buddies. While on a fast, I spent many hours on Yahoo Answers.