I also had a
stress test this week. and like you, I had a Thallium
test as there is a national shortage of myoview. Any strong headache should be reported to your doctor, and at the same time, several aspirin tablets should be ingested.
That being said, I'm betting you were told not to have any beta blockers, in my case no slow-release nitro pills, and certainly no caffeine. Withdrawing from these products all could result in a headache.