There are a lot of reasons why symptoms can show up so late after the original injury. Illness or any immune system compromise would be first on my list. A cold, the flu, allergies, a period of high stress from work or relationships, getting drunk, etc. can all put stress on the brain. Even good stress such as happy events, can cause the brain's stress load to increase. Attending a concert or other high stimuli event can cause a serious crash.
The progression of a concussion usually peaks within 2 to 3 months. But every injury is different.
The best treatment is the same as all PCS. Good nutrition, low stress, good rest, etc. Avoid more than a single serving per day of alcohol and/or caffeine.
The CT scan will probably be negative, as will most imaging after a concussion.
There are no ways to prognose a time line for recovery.
So, be easy on your brain. It has had if share of trauma.
My best to you.
Mark in Idaho
"Be still and know that I am God" Psalm 46:10