Tink - You and I work in the same type of classroom (special needs, preschool). As I'm sure you know, it's not an easy job even on days when you're feeling great. I'm a paid staff member and on days when I'm not feeling well, I take a sick day and call in for a sub. It's not good for me, the kids, or the other staff members if I'm not well enough to do my best. It can even become a safety issue since we often have to chase kids who run away from us, break up fights, and give 100% of our attention to situations in which someone could get hurt.
I would urge you not to go to work on days when you're not feeling your best. Your classroom team will compensate for your absence by getting someone else to help, or dividing up tasks in a different way. A volunteer job is important and I admire your dedication, but it's not worth risking your health.