If we are talking about the A, In the USA we say "i have a toothache." We do not leave off the a.

Russian point of view: we have one head and one stomach but too many teeth( who knows which one is aching )

This is just writing tradition, was said in tutorial. No logic :) http://www.classes.ru/grammar/19.hornbi/html/1_21.html (in Russian)

errr...what!!!