If you are going to do a colonic which isn't really necessary do it early on. The fast will go smoother if you get out all the old food that's sitting there. Ideally the 1st 3-4 days drinking Epsom Salt in water( 4T epsom to 3Cup H20) will clean your colon just right.
Once the 1st layer of old food is out you will start to feel the fasting high. If you keep at the fast your body will naturally eliminate the excess toxins. If you are real sick then extremer measures might be in order.
People get sick while fasting because the old stuff is sitting there. Make it easier on you colon by eating raw several days prior and ideally the elimination process above.
What kind of juice are you fasting on? Fresh Squeezed OJ is ideal has everything you need and is simple.