"Listen to your body?"

People always say "listen to your body" when we talk about doing this cleanse.

I am not sure how to discern the "good" discomfort from the "bad." How do I really know what my body is telling me? Right now I am pretty miserable. (Day 2) Headache, intermittent cramping, feeling spaced-out at work, unable to really type straight,really.

Last time I did this cleanse I felt pretty good till Day 6, then started feeling bad. So I stopped at Day 7.

Obviously, simply feeling bad is not necessarily your body telling you to stop or to try something different. So what is?