Infections/Illness
It is understandable that if you are suffering from an illness or infection, chances are that along with the rest of your body, your throat may also not feel 100% percent. So give it a rest just like you would give your body a rest and some 'time-off' to recuperate.

Stress
Believe it or not, Stress can have a major effect on your throat. You may feel tension, or tiredness or tightness. Each individual is effected differently. Deal with what's causing you stress and see or rather feel the difference.

Medication
Medication can have all sorts of effect on your throat. They may either dry you up, or make you too drowsy to want to get up on stage and do anything etc.

Menstruation/menopause
Problems with voice quality are often experienced during puberty, menstruation and menopause. Vocal cords seem puffy and there seems to be a reduction in their muscular tension. On top of this, the lethargy and lack of energy during such times will you with poor breathing control over the diaphragm. It would be silly to advise no singing before and during a period because of this however, if your symptoms seems particularly uncontrollable, then that would be the best thing to do. However, singers can often work around these problems by making fine adjustments to the voices.