As the CT scan
of brain is normal probably organic causes can be ruled out specially at this stage chance of brain metastasis
is high. You should have to note the type, intensity, frequency, duration and relieving/aggravating factors related to headache. If the headache is associated with vomiting
then this should be carefully investigated. other type of headache like cluster or tension headache
should also be kept in mind. You should take a neurology opinion.