Got hit in the back of the head with a foam dodgeball 10 hours ago. no severe headache. still need to be concerned about brain damage?

No knockout/no pbm: The trauma that knocks you out, even briefly, has produced a concussion. This frequently includes short term memory loss, dizziness & nausea & headaches. If the trauma wasn't enough to cause even brief loss of function is unlikely to ever yield brain damage. Headaches, dizziness, nausea & vomiting may indicate you had more of a hit than you remember( memory loss is a sign of concussion).

It depends: What symptoms are you having? Did you lose conciousness? Do you have any memory loss for the time before or after the injury? Even if you have a normal head ct, you could still have mild traumatic brain injury, or concussion, which can cause longterm problems with memory, cognitive function, headaches, fatigue and dizziness. See a doctor if your symptoms worsen or do not improve in a few days.