Chlorous acid (MMS) and chlorine dioxide oxidize iron ions. Since the blood needs iron in order to carry oxygen, your ability to carry oxygen can be reduced. This can make you feel tired.
Many people think that feeling tired is a sign of healing, but it most likely is an early sign of oxidative stress in the body. Since there are no studies on this, you will have to take it up with a medical professional that understands how body chemistry works.