How come I have a really dry mouth today?

Dry mouth. There are many conditions associated with dry mouth. It is not a disease, but a symptom of a medical disorder (e.g diabetes) or a side effect of certain medications, such as antihistamines, decongestants, pain killers, diuretics and many others. It could be a single episode and no further treatment is needed.
Dry mouth. There are many things that can cause a dry mouth, from medications, to dehydration, to anxiety and fear, to neurologic and other diseases affedting salivary glads. I suggest you see your family doctor to investigate further.