I got hit in the head with a tennis racket and now have a head ache. is that bad?

Not unexpected. This type of trauma can bring on intermittent headaches even a week after the event. By itself, the symptom is neither worrisome nor unexpected.

Not unexpected to suffer a post-traumatic headache under these circumstances. It becomes "bad" if the headache worsens, nausea and vomiting ensues, your vision blurs or doubles, dizziness results in falling, you get confused and disoriented, suffer seizures. Any such symptoms should prompt a doctor's visit.