OXYGEN HANGOVER?

All,
I fly from an airfield that is over 6000 feet elevation, therefore I
use oxygen on most flights. I have found that I get a headache that
lasts for several hours following every flight. I use an EDS system
and have tried several different settings; everything from its maximum
setting which delivers oxygen when I'm sitting on the ground, up to
having it begin to deliver oxygen starting at 10,000 feet. When we
first moved here, it took a day or two to acclimate and the headache I
had then is the same as my post-flight symptoms as far as I can tell.
Has anyone experienced similar symptoms? I feel fine in the air and
immediately after the flight, but a half hour or so later I get the
headach. The only thing I can figure is that my physiology must be
sensitive to changes in oxygen levels. Any suggestions on something I
may be doing wrong? Something I can do to prevent my oxygen hangovers?
Thanks.