<font color='#000080'>It could be from blood pressure being elevated during the dive.
As you may notice if you need a pee at the start of the dive, you will be desperate by the end! This is cos the body tries to get rid of what it sees as too much water by the only way it effectivley can - making you pee. Make sure you are fully hydrated before you dive to avoid getting dehydrated - a major contributing factor to many bends.
I had a stonking headache two days running in scapa - but i know this was due to the bloody ferry coming in at 3am and fannying about with the bow thrusters which sounded like someone hoovering the deck of the boat.
Our ex-DO used to get a headache whenever he worked hard underwater - he used to say it was a CO2 headache, but as far as i know you must be doing something wrong to get one of these (but i may be wrong) i think they are more likley with full face masks.
Is the neckseal of your drysuit tight? This can cause pressure on the vagus nerve (part of the parasympathectic nervous system, i think its the 9th cranial nerve...not sure there either) which can cause all sorts of havoc with the body, and also bugger about with the blood pressure in your head.
Lastly, do you wear glasses? You may need a prescription mask if you do, as eyestrain may cause probs too.
However, a headache can be a symptom of a bend, so i would suggest you talk to the DDRC, they are quick to reply and very helpful via email.
ddrc