There is a great deal that has been written about cognitive issues after cancer treatment, commonly called "chemobrain." That is a less than ideal phrase as the problems may occur, too, for people who have not received chemotherapy. Here is the bottom line: These problems are very common, hard to tease out the causes, and they improve for almost everyone once a little time as passed since treatment.
For many people receiving active cancer treatment (carefully not saying "chemotherapy" although it is more pervasive for those who do), the problems are most often described as difficulty with memory, a certain fuzziness or blunting of mental sharpness. I have heard women describe looking at a computer screen of figures or documents that are ordinarily a routine part of their work--and having no idea what they are seeing. Many of us experience some trouble with word finding and memory (e.g. find ourselves in the kitchen and can't remember why we are there).
No one really understands the etiology of this problem. Certainly all the drugs that are circulating in our poor bodies, especially those powerful chemo ones, contribute. As do stress and probably less than good sleep and distracted thoughts and fatigue. This is an excellent article from ASCO's CancerNet; I give you the beginning and a link:
Attention, Thinking, or Memory Problems
This section has been reviewed and approved by the Cancer.Net Editorial Board , June / 2013
Cognitive problems, also referred to as cognitive dysfunction or “chemo brain,” occur when a person has trouble
processing information, which includes mental tasks related to attention span, thinking, and short-term memory.
Up to 75% of people with cancer experience cognitive problems during treatment, and up to 35% have issues that
continue for months after treatment has finished. These difficulties usually vary in severity and often make it hard to
complete daily activities. People who experience serious cognitive problems are encouraged to talk with their doctor,
nurse, social worker, or another member of the health care team about ways to manage these issues.
Signs and Symptoms
Relieving side effects, also called symptom management, palliative care , or supportive care, is an important part of
cancer care and treatment. Talk with your health care team about any symptoms you or the person you are caring for
experiences, including any new symptoms or a change in symptoms.
Cognitive problems include difficulties in many areas, such as:
Trouble concentrating, focusing, or paying attention (short attention span)
Mental “fog” or disorientation
Difficulty with spatial orientation
Memory loss or difficulty remembering things (especially details like names, dates, or phone numbers)
Problems with comprehension or understanding
Difficulties with judgment and reasoning
Impaired arithmetic, organizational, and language skills (such as not being able to organize thoughts, find the right
word, or balance a checkbook)
Problems performing multiple tasks (multitasking)
Processing information slower
Behavioral and emotional changes, such as irrational behavior, mood swings, inappropriate anger or crying, and
socially inappropriate behavior
Severe confusion (delirium) 
The severity of these symptoms often depends on the person’s age, stress level, history of depression or anxiety, coping
abilities, and access to emotional and psychological resources.