Ok, maybe my Googling-fu isn't up to task, so sorry if this is a dupe, but I can't seem to find anything to help.
I'm training for my first century, but I'm a full time commuter. I ride 9 miles a day, twice a day, and I usually get my heart going. I don't have a HRM, but I know that I average 16-18 mph during those rides, and my resting pulse is about 52 bpm. I think I'm in pretty decent shape, but I don't have a lot of experience with really long rides. I've ridden most of my life, and I used to do fairly regular 30 mile weekend rides without incident.
Anyway, this weekend I did one of the longest rides I've ever done, 50 miles. I did it in just under 3 1/2 hours with a 15 minute stop at 25. I had a big dinner the night before of pasta to give me some energy, I had a good breakfast an hour before I rode, and during the ride I consumed 50 oz. of water, 24 oz. of sports drink, and a Clif Bar. I felt good but drained afterwards (had a pretty severe bonk that I think was the onset of heat stroke on my last 50 mile attempt a month and a half ago, so I was really careful this time). I weighed myself before and after, and I lost about a pound or so, so I may have been a little dehydrated. I did urinate immedately after the ride, though, and it wasn't overly yellow.
Since then, however, I've been getting these nasty headaches with ANY exertion. I walked about 2.5 miles the day after (leisurely) and my head was pounding. I avoided my commuting ride today to recover more, and my head started throbbing after running across the street to catch a bus. Is it possible I'm still deficient in sodium/potassium/whatever? Is it just possible that the exertion of riding longer (I have done sporadic rides of 30-40 miles before, but my previous long was 42) caused this?