Am I sick? Or was it just an off day?

Really, really, really hard day with the trainer today. I don't know what it was, but I struggled through the whole thing.

We did arms the whole time, and in between circuits of arm exercises, I did a set of cardio (20 jumping jacks, 10 mountain climbers and 15 toe taps on a bench, two times through). It got to the point where I felt like I physically couldn't pull anymore weight. My face was bright red. My head was hot. I was breathing heavy. It was really discouraging.

I woke up with a bit of a sinus issue, and several people I know have been sick. But if I don't have full-blown cold symptoms, can the virus in its early stages still have that kind of effect on my workout?
Also, and this is kind of gross, so I apologize, but I noticed I was only sweating from my head. Not my back or anywhere else, which is usually the case. Is this a sign of illness? Another thing: No matter how much water I drink, my mouth is dry and sticky. Am I getting sick?

Less than an hour after the workout, I feel totally drained. Just knocked out. And I'm just wondering what the problem was today.

I guess, at the very least, I was burning some serious calories. Still, I wonder what happened.