Normally I don't ride when I feel sick, have headaches or body aches, or if my allergies are bad. I am pretty resilient, so I am not sick often but when I am it is normally worse than a common cold. Also, I won't be able to ride my best while sick, so why risk having a bad ride or getting hurt?
I am in a similar situation, not feeling sick sick but not 100%. Tomorrow I have a doctors appointment for my headaches and will miss school. Free time = horses for me, so hopefully I can get my mom to let me ride :)
But, when I stay home from school on those AMAZING days, I end up going out by the horses to "get fresh air". And then normally one needs to be brushed, but since I already have them out, and the saddle is right there.... Just make sure to be finished riding by the time anyone gets home :)