I have a concussion and i hit my head hard, can it make it worse?

Yes: Persons who have had one concussion are more susceptible to a second concussion from equivalent mechanical force to the head than someone who never had concussion. But if the 2nd hit didn't cause a new concussion, probably no damage done. But be careful. Too many blows to your head, even if sub-concussive can cause something like the "punch-drunk" syndrome. Once this happens, it cannot be undone.

Yes: need to be careful after a concussion. After a concussion, a second concussion may result in a 'Second impact syndrome', a potentially life threatening condition. If you have not been made worse, rest both physicall and mentally until symptoms resolve.