Woody!
I experienced the same symtoms as youself on Saturday!
This was on a century ride.
I believe my big mistake was not to take any drink with me (to save weight) foolishly, and rely on stopping at stores to purchase drink.
Stupidly, as it turned out, i failed to stop in time at any where i was passing through, until it was to late.
About the seventy mile distance my legs just gave out, every uphill for the final thirty miles was sheer torture, I felt light headed and nauseous.
I Had previously completed the same circut on 3 previous occasions, but i think this time I did not make allowances for it being at least 10 cent`ade warmer.
And to compound my misery, 30 miles into the ride i was stung by a wasp!, this was sore and irritating for the remainder of the ride(three days laterand it still is)
Guess i`ve learned my lesson the hard way to make sure you keep yourself well lubricated with fluids, on long hot rides.
Take care
Willi`c