I am recovering from a sinus infection, and aside from two days that I took off from work when just getting up to go to the bathroom made me exhausted, I rode every day of the thing.
I am a firm believer in the idea of "sweating out" an illness if at all possible. I remember reading somewhere that if you have a cold/sinus infection/flu type of sickness that has most of its symptoms above your neck, then you are fine to exercise as much as you want to. It is only when you get the coughing gunk up, sore lungs, achy joints type of "below the neck" symptoms that you should lay low.
Yesterday I was still feeling pretty darned tired and clogged up in my sinuses, but I went out on a ride that turned into 50 miles. I found that the fresh air, and the feeling of my muscles moving made me feel much better and today, I almost feel back to normal.
I would just go a little more gently than normal and see what happens.
Just my two cents.
Kat