Hello As you said the headache is intense with involvement of neck and shoulder & CAT scan being negative, I would like to advise you that you need to get a cervical imaging done starting with a radiograph to rule out a cervical disc bulge causing cephalalgia & in addition you need to describe your headache in terms of whether it is unilateral or bilateral, aggravated or relieved by somthing, any vomiting accompanying headache, fear of lights or noises, if nothing comes out you may need an MRI Brain & cervical area. Meanwhile you can take analgesics for temporary relief. Hope I have answered your question.