It'e what we call die-off. It is also know as herk or more commonly the herxheimer effect.
We are detoxing and we do too much, too quickly, our bodies are unable to process the debris/waste, or efficiently enough. The recommended method of dealing with this is to cut back on the cleanse or limit the load of toxins we are asking our bodies to deal with. People can become so auto-intoxicated that they have serious healing crises.
This is another reason why we work so hard to push the waste out in the Master Cleanse, so we do not reabsorb it and to keep making room for more to be processed through the skin, bowel, gallbladder/liver, and kidneys....
Here are a couple of links to give you an idea of what I am describing.