A few things may be happening here.
Your body is working extra hard to get rid of all the built up toxins, so that may be one of the causes of your fatigue.
It is very common for our detoxers to report increased energy on the detox, but for some that doesn't show up until later phases.
Stay the course and you will notice the benefits later on. This is a common occurrence, but it passes with time.
Another reason might be that you're not eating enough calories. The foods we eat on this detox are very low calorie, so it's possible for you to be eating larger portions but still consuming fewer calories than you would during a normal meal.
Make sure you're eating enough to be sustained throughout the day.