Hi, Love/Hate09. ATM, I would consider the side effects of any medications you are taking and the quality of your sleep. With some medications, side effects may start to develop due to the way an individual's body metabolizes the meds.
For instance, there may be a change in the person's kidney and/or liver function that could be the cause concentration problems.With the antidepressant I am on, if the dosage is too high, it causes anxiety that results in my poor concentration.

And the other point is sleep. A person who suffers from a sleep disturbance could incur similar concentration problems.

These are from my personal experiences and I am not rendering any health care diagnoses or treatment, just possible avenues to explore.