I just came off a 10 day sick period. I had something resembling the flu, cold, cough all rolled into one. I felt horrible. I was also mad because I couldn't workout. I wanted to, but I was simply too ill. I think this is a good guage of whether you should or not.
With a normal cold or headache, I'll push through and do my workouts anyway. It always makes me feel better. But this time I just couldn't.
I am almost back to normal, but my lung capacity is still below par. I had the most horrible cough you've ever heard, so it's no wonder I still can't catch my breath like I should.
My advice, go with your instincts, your gut feeling so to speak. If you don't have the energy or the will to workout, your body is telling you to rest. Make yourself take a few days off. Even though I hated it, I think it was the best thing for me.
Hope you feel better soon!
Jeanne