The pituitary gland controls the activities of a number of other hormone-producing glands. One such gland is the thyroid. The two adrenal glands are another example. A disorder of the pituitary gland can thus set off abnormalities in the function of other glands.
Should the function of these glands be disturbed by the tumour of the pituitary gland or the consequences of surgery, they could produce the findings noted currently in your mother.
Another cause for these disturbances could be disordered concentrations of sodium and potassium in the blood stream.
Finally, surgery on the pituitary gland may, at times, be followed by the development of hydrocephalus - abnormal collection of water in the cavities of the brain.
I suggest you get the doctor who did the operation to see her again. Tests to uncover the precise cause of her present illness can be identified and treated.