What Causes Persistent Headache For Three Days In An Elderly Person Who Recently Finished Taking Casodex For Grade 1 Prostate Cancer?

I am feeling on and off headache for the past 3days. It start with headache and neck pain The neck pain had subside but the headache is still on I am 73years old and had finish taking Casodex for two weeks and I have grade one prostrate cancer what is the cause of the continue headache