Hi I've had an issue with headaches after swimming underwater in a pool at only 2m depth. 25 yrs ago I was swimming a lap underwater 25m, then a few breaths and another 25 m and I had an strange experience of intense throbbing headache at the back of my head near the base of the skull immediately after surfacing fom an underwater lap. It came back 2 weeks afterthat initial occasion whilst doing some circuit training but never after that. Since then it hasn't happened. I have been spearfishing quite a lot and done a basic free dive course in the last 15 years as well as lots of other sports with no intense headache issues. A few days ago after some time off I went to the pool and did a 25m followed by a few breaths and another 25 m. I came up to the same intense headache and the throbbing lasted 5 mins before settling down to a fuzzy foggy headache for 36 hrs and general lethargy and weakness. I went to Dr 36 hrs later and had scans... no aneurysms or vascular disease. Interestingly my blood report showed higher than normal CO2 levels? Headache has gone but the dr could not give a diagnosis? I am now 46 and consider myself fit and healthy and not at all overweight. Has anyone had anything similar or knows what it might be? I might not be exhaling enough when I breath hold? Maybe holding air in lungs? Any experiences?