Headache off and on for 2 days and constipation. otherwise feeling fine then all of the sudden had violent shivering for at least 30 min. not cold?

Sounds viral!: Have you measured your body Temperature??? Shivering is a common symptom of a VIREMIA (Viral multiplication and blood spread) whereas a headache is often a prelude to a viral infection! Hope this helps! Dr Z

Usually a fever: A high fever usually accounts for intense shivering or being very cold which you aren't. Is there any way you could have mistaken the shivering for a seizure? (though it would be very unusual to have a seizure for 30 minutes). The preceding headache might suggest something going on with your nervous system. Rarely violent shivering is a manifestation of a panic attack. Evaluate urgently if recurs