Relating to the seal, if it is latex, make sure you have it down as far as possible on your neck. If you think it is comfortable otherwise, that might do the trick without having to resort to cutting the seal.
Since you mentioned that you were working hard another time when this happened, it could be CO2 retention. You need to slow down, and breathe slowly and fully. If that works, that was the issue.
Maybe you are just getting a stiff neck. Make sure to stretch before you suit up to go out diving. This might take care of the problem especially if you notice it happening in colder water only.
There are just too many possibilities. There could be other serious medical conditions that could be causing this problem. Especially if this is a new issue for you, you really should get checked out by a physician instead of relying on Internet advice, even my Internet advice, for what to do here.
Getting back to your post subject though, I would, in a non-physician sort of guess, think that the added oxygen in an EANx mix would not be the cause of this. (I wonder if it is possible that oxygen is causing some kind of vaso constriction that could contribute to your headaches. I wonder if this would make problems for potential migrane sufferers. Even if it is possible, I would think this could not be a real factor unless you are really on high mixes though.) However, other than this kind of thing, I am guessing that the CO2 thing would be more of a likelihood if you don't have some sort of medical condition that causes this to happen or if your neck seal isn't the cause.
