BAD HEADACHES AFTER EXERTION

For the last year or so everytime I exert myself I end up with a bad headache. I told my doctor and he said, it was caused by stress. I have been under stress before and never ever gotten headaches like these. They start either on the top of my head or around my eyes or in my temples. I got my blood pressure checked and it was 120/80. These headaches are starting to worry me. I told my doctor I thought I might have a brain tumor and he just shook his head and laughed. Does anyone know what could be causing these headaches ? I got my blood work done and it came back normal. Please Help.