During a 150-mile 2-day cycling tour ride last summer (mid-September), I was at about the 70-mile point and starting noticing alternating chills and heat. I had been drinking and eating (sprots drinks, water, crackers, and a PB & J at lunch, plus several gel supplements throughout the ride.
I'll add that the weather was hot (85-90 deg, full sun, no wind (other than relative wind)).
I recall that at the 60 mile point, I felt a little "distant" (not dizzy or incoherent - just "distant").
After the day-one ride completed (75 miles), I went to my hotel room where I continued to feel this alternating pattern. I also had that same feeling that you might get after being outdoors at the beach all day (OK... I had been outdoors riding all day... that might account for that). I got something to eat (meat, potatoes, bread, etc.) for dinner.
I never got a headache or anything, but felt worse and worse (but still no headache, vomiting, or cramping). This seemed to peak about midnight.
At 3 am, I woke up and felt much better. At 6:30 am I got up and felt OK enough to ride again. I felt better as the return ride progressed. I finished strong the second day, but did experience one brief chill set around 50 miles, then all was fine after.
At the end of the ride, I felt great. No problems.