thanks for writing to us.
A high blood pressure
due to heat pack application on neck is unlikely, unless you are using too high a temperature which is causing discomfort to you.
Our blood pressure goes on changing continuously and is dependent upon level of physical and mental activity. Since you have headache for last 3 days , the rise in blood pressure (BP) can be explained by this to some extent, as headache is a kind of stress
and a reactionary rise in BP can occur. however if the higher level of BP persists even after resolution of pain or if your BP levels are getting higher and higher or you have a new symptoms, please consult your doctor.