I'm confused about this particular sentence, and other ones using ache,I know that 'I have a headache' is correct and works, but it sounds a little weird to me if someone says 'I have a stomachache/a toothache/a backache/an earache'. I feel that with the exception of headache, these should all be uncountable, does anyone agree with me or am I barking up the wrong tree?
While it is common for one to see the words, "headache", "stomachache" etc., spelled as one word, they are actually closed compound words.
Take out the noun adjunct (e.g., head, stomach) and say the sentence again without the indefinite article:
"I have ache."
So ache is in fact a count noun, with a noun adjunct modifier attached. Hence the indefinite article.
Hope this helps?