This is probably country specific but in the UK you should be able to take the time as sick leave for however long you need, provided you have a doctors note.
"Taking Sick Leave (.gov)"
The idea of having a limited number of sick days has always seemed ridiculous to me, but I'm guessing that's sick days you don't have a doctors note for so I would suggest you get a GP to sign you off from work. If you need more days for a different illness later in the year do it again. Assuming you're US:
In the US, there is no requirement that businesses offer paid sick leave to their employees. The only thing they must do (provided the business employees more than 50 employees) is to allow staff unpaid time off for medical leave, or to care for a family member. How does sick leave work in the US
Which would imply that your company can't stop you taking time off if you are unwell, you just may not be paid. How many days of paid sick leave is an American worker entitled to
Concussion can be really serious and if you are still experiencing the effects days later then I would suggest seeing a doctor in any case.