Have you ever concluded that you might have had a fever? As in, you didn't think you were sick at the time, but in retrospect the evidence suggests that maybe you were?
Last night, my face felt hot, I had a headache, it was difficult to think straight, and I felt the need to lie down and stay down before it was even 9 pm. At the time, I just concluded it was the heat, but the air conditioning had been on and I had a fan going once it was off, so it wasn't actually all that hot inside. In addition, I wound up turning off the rotate function on my fan because my forehead would feel hot the instant the fan turned away from it.
I dunno, maybe it really was just the heat outside getting to me. I guess I'll never know for certain, but I'm drinking plenty of fluids and staying out of the heat today.