This is not a glamorous topic, but hopefully useful. Many people have allergies which they may or may not know about, and these can affect singing, sometimes drastically.
Allergies can cause postnasal drip, inflammation, congestion, swelling, discomfort, disorientation, skin problems, asthma, and other symptoms. Many of these can affect the voice. The two vocal folds, the source of all vocal sound, are very small (up to 3/4 inch or 19 mm long at rest) and covered with mucosal tissue. Anything that can affect mucosa (such as many allergies) can affect the folds. There can also be secondary effects such as postnasal drip falling onto the folds causing irritation, or stress due to compromised breathing. Whenever the mucosa is disturbed, vocal function will be affected.
I encourage any student with recurring problems that might be allergies to get tested. Knowing what your allergens are is half the battle. If it turns out that allergies are not the cause of vocal difficulties, you can explore other options such as acid reflux, vasomotor rhinitis, etc. Getting the body as calm and nonreactive as possible not only leads to better singing, but to better health and comfort.
Having said all that, it is also good to know what adjustments to make for the days when your body feels different. Being human, we will not be in exactly the same physical condition every time we sing, and learning how to deal with that is part of being a singer.
If in doubt, get tested!