Damn, warming up is something I took for granted that everyone does.
If you feel sick, don't workout. It won't kill you. Take me for example, I had a nasty stomach flu on Friday-Saturday, puking my guts out. On Saturday night I felt better, and on Sunday I felt good enough to hit the weights. Even though my muscles felt weak, I was still pushing the same heavy ass weight that I was pressing before I got sick. Now in my case, I WAS sick, bad. I didn't just feel sick.
Chances are when you workout while sick, you'll have a shitty workout, then beat yourself up about how you didn't lift heavy enough and don't know why.