What you describe may require prompt attention. Please stop the "breathing exercises" immediately, contact your doctor now, and request consultation with a neurologist, to be seen as soon as possible.
The type of headache you have experienced is called a thunderclap headache, which is a sudden and severe headache, maximum in intensity at the onset. This type of headache has been described with hyperventilation and exertion, due to blood vessel spasm in individuals with normal brain circulation.
However, it has also been described in people with subarachnoid hemorrhage in association with cerebral aneurysms. Aneurysms may be present for years with only slight, recurrent leakage. A thunderclap headache can also be a nonspecific presentation for a variety of other vascular and nonvascular central nervous system processes.
Your headache could be due to the breathing exercises, either because of hyperventilation or, without being aware of it, you are engaging in what is called a Valsalva’s maneuver. This maneuver can raise pressure throughout the body, including in the cerebral vessels and can also transiently reduce blood flow through those vessels.
Sorry to provide such a worrisome response, but we do wish to convey the urgency of your situation.
Please give us a follow-up to let us know how you are doing.
I saw a D.O. this week for some crainal work, and she said the cranial sutures between my nose and forehead were jammed together. After her work, it was like someone had opened the top of my nasal passages about 3 times greater than what they were, and it was much easier to breathe, and less headache, too.
It hasn't held, though, and I'm still not convinced that was the entire problem - that there isn't something deeper going on. So, I will get it checked out.
THANKS for your advice!