Since you had a subdural hematoma and craniotomy, even though you do not have a headache, consult your specialist regarding a re-bleed, or a raised intracranial pressure or high BP. Electrolyte imbalance and dehydration is another possibility. This can be corrected by drinking water, clear soups, juice etc.
Since a confirmed diagnosis cannot be given on net please consult your doctor regarding this. Hope this helps. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.