Are feelings always a noun? For example, "headache" or "nausea". Why I am asking this question is because I always have a problem with countable and uncountable nouns. First job is identify the nouns. Thus, I am asking is feeling a noun always.
A noun is generally defined as "a person, place, thing, or idea". So a thing that you are feeling or can feel must be a noun, because it is a "thing". So yes, "headache", "nausea", "happiness", "depression", etc, are all nouns.
The state of feeling a certain way is an adjective. So in "I feel nauseous" or "The happy man left the room", "nauseous" and "happy" are adjectives.
"Feel" is a verb. It's present participle is "feeling", as @Damkerng T. mentioned in the comments.
However, "feeling" is also the noun. It is countable. Here are some examples of its usage:
I have a feeling tonight's going to be a good night.
It's an uncomfortable feeling, knowing that it could all be over soon.
I hate feelings of pain; headaches, nausea etc.
I couldn't resist citing this welll-known phrase:
I have a bad feeling about this
(just to make an example of feeling used as a noun)