Certain people with certain conditions can't fast, not sure if you are one or not.
".... you should not fast if you have liver or kidney weakness or disease, or are extremely frail, malnourished, anemic, or exhausted. You should consult a doctor and be under his/her care during fasting if you have a weakened immune system, severely high blood pressure, medication-dependent diabetes, or weak circulation causing frequent fainting." http://www.allaboutfasting.com/who-can-fast.html