What Causes Severe Pain At The Back Of Head During Workouts?

I m a 48 year old woman in reasonably good health, I am about 60 lbs over weight, I have recently started working out, one night is cardio and one night is weights, as soon as my breathing gets labored and my heart rate goes up I get the most intense pain in the back of my head right where the neck and skull come together. It so bad it effects my eye sight and the pressure is so intense it feels as though my head will explode, and it takes hours for it to stop after a work out. I have no idea why this is happening