For some people yes that happens. However it is always best to consult with your doctor about any health concerns. It is most likely one two things, detoxing or too much sugar in your blast. When you body is getting a lot of nutrients it can begin getting ride of toxic materials that have been stored in your body. This is a good thing! Detoxing side effects can cause fatigue, headache, low grade fever and even sweats. It passes once your body has cleared most of the toxins from your body. Too much sugar can cause cause fatigue from the sugar/insulin crash that happens after eating or drinking a food that has a lot of simple sugars in it. Such as too much fruit in a NutriBlast. I recommend using mostly vegetables and a few fruits. If you are just starting out try 1/2 veggies and 1/2 low sugar fruits. Low sugar fruits are berries, green apples, peaches, pears and plums. You can also use bananas if they are green on the tips and bright yellow (like this they have about half the sugar content). I hope this helps. Many blessings Wally Bishop C.N.C.