Why do we say " toothache" but "A stomach-ache, A headache"? Please tell me, why?

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

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

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

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errr...what!!!