Brain fog or mental clouding is a general sensation of unusually poor mental function, associated with confusion, forgetfulness
and detachment. A number of medical or psychiatric conditions and treatments can cause such symptoms, including: chemotherapy treatment, chronic fatigue syndrome, fibromyalgia, heavy metal poisoning, hypothyroidism, major depression or schizophrenia.
Otherwise, this may just be allergic reactions or sinus infection. Are there other signs or symptoms present?