A lot of things can cause severe headache, but the most common are extreme stress, emotional distress, or underlying medical/health conditions.
If you subject your body to intense stressful conditions either physically, mentally, or emotionally, you are likely to have a headache and this type is known as a primary or standalone headache.
You could also have these headaches without stress, constantly too and this is usually a way of your body telling you there is something wrong somewhere, medically.
To know which is which, you would need to go for a checkup to get a proper diagnosis, especially if you haven't undergone any kind of stress lately.