Why do I get headaches?

I've been a lap swimmer for years but in the last couple of years have increased my distances substantially compared to past years. Unfortunately one of the negative by products has been headaches after I finish my workouts. This usually occurs when I swim longer then 90 minutes. During the swim I'll feel great. No problems after quitting. Then about 30 to 45 minutes after I finish I'll feel the headache coming. 90 minutes after the swim ended I'll have a nasty headache. I take ibprofene for it and it goes away after an hour or so. I thought it might be dehydration, so I started swimming with a large water bottle at the end of the lane and I'd take a quick drink every 1/4 to 1/3 of a mile. That didn't seem to help. I added a 20 oz bottle of Gatorade, along with the water, doesn't seem to have made any difference. I went to my family doctor about 8 months ago and had a bunch of blood tests done to see if something would show up there, nothing. No blood sugar problems or other "inbalances". So I keep swimming and keep getting these damn headaches. Anyone have any clues as to what could be causing them or possible remedies? Thanks,