i cant just say "no i cant fight i got ko'd recently".
Yes you can. Who's hindering you from pronouncing exactly these words¿ Look at the professionals. After a KO often they are not allowed to fight for several months - and in pro fighting business (fighting) is usually valued higher than health. So if you're not a pro fighter (which your way of questioning doesn't suggest) you should keep it easy for a while to your own interest.
Btw. the brain itself cannot feel pain. So pain isn't a reliable indicator on how much damage the brain has taken after a trauma. That's something you only can find out via MRI.