In American English, stomachache is a countable noun, which always takes a determiner (the, a, this, my, etc.) when it is singular.

You can use the expressions “have a backache” or “experience a backache” in American English to refer to a continuous pain in your back.

In American English, we normally use the expression “have a toothache” when experiencing a pain in or about a tooth.

We say the expression “have a headache” to refer to a continuous pain in the head. The expressions “have headache” and “having a headache” are incorrect.

In British English, you can use the expression “on holiday” to mention a period of time away from school or work. This is the equivalent of the American “on vacation”.

In American English, we typically use the expressions “on vacation”, “being on vacation”, or “going on vacation” (without the article “a”) to refer to a period of time (away from school or work) to relax or travel.

When talking about the first meal of the day, we say “have breakfast” (not “have a breakfast”).

We commonly use the expression “have dinner” (not “have a dinner”) to refer to the main meal eaten either in the middle of the day or in the evening.

We use the expression “have lunch” (not “have a lunch”). However, when using an adjective or a compound noun, we add a determiner (typically the article “a”) before the noun.