Hi Jacqui. Often the body will respond to our exercising by providing more energy and it's no problem at all. If your body isn't doing this for you it's probably a sign you were exercising a bit too much. Each fast is different though and as you clear out more toxins and your body becomes healthier more energy will become available for other things than detox and healing. It's a matter of experimenting really as there are no hard and fast rules. Some say Active fasting is nearly always better while others recommend resting all the time. I think listening to your body as much as possible is the best way to go. Try exercising less, and if that doesn't work see if you are better off resting altogether. How your body responds to either nobody can tell it's just a matter of try and see.
One thing that might help also is making sure you allow more time to recover after any vigorous exercise. Warm down/stretch if necessary then actually make yourself lie down for a while to rest, I think it's better to do this even if you don't feel completely worn out while fasting. Even a short period of complete rest can make a difference and it's something we wouldn't usually think to do.