Post-Concussion Syndrome (PCS) refers to a range of reported symptoms which persist after a brain concussion event - sometimes for months or even years.
PCS is estimated to affect 15-30% of people who have had concussion injuries.
Symptoms can be physical, e.g. nausea, dizziness, drowsiness etc, or psychological, such as anxiety, mood-swings, irritability etc.
The causes are not yet known. See : Wikipedia.
There may be some as-yet-undetermined physical damage to the brain, and/or factors associated with the trauma of the event.
Note : PCS is now under close scrutiny due to the discovery that repeated, though relatively minor, concussion events can cause cumulative damage - especially in sports, e.g. boxing, rugby, soccer etc..