as77, when I did my GTT, I had similar issues. I didn't have the spike you felt, I was fine. But by the time they did the 2 hr labs, and I left, I was in a bad way. I beelined to a burgerking and got a coke and a burger. Chugged the coke, all but swallowed the burger whole.
The crash after the test is normal, don't sweat it.

Even my DH, he had to do it all last week, and he had it too, and he's healthy as a horse!