Delay
One thing a lot of people don't seem to be aware of is that injuries can often times show up significantly delayed. A concussion from a head injury can sometimes only start showing symptoms after a week.
There are also cases where these injuries continuously worsen over a period of multiple weeks before the person realizes the original cause of said injury.
If you're in a country with "free" healthcare, just go for a checkup since you're already paying for it.