Uneasy at back head after having water.

I drink water around  0.5L with empty stomach on everyday morning. Now a days I am observing mild pressure at back of head after drinking water after around10-20 minutes. Some times circle (white horizontal dashed circle) in vision for short span of time (with in second), noticeable when blinking eye. No issues if I ate anything immediately after drinking or no issues if I drink after eating also. Whats happening with my head ? Around 6 months back saw some strange symptoms, pulsating arc at corner of both eyes near nose and then headaches. Last episode was got camera like white flashes in eyes. Went to doctor, used amitriptyline and b12 tabs, those were gone. This may be related to above problem. But there is a problem when having water with empty stomach. Please assist.