pain vs unpleasantness

pain and unpleasantness both are nouns.

Word	Noun	Adjective	Verb	Adverb
pain	Yes	No	Yes	No
unpleasantness	Yes	No	No	No

As nouns, unpleasantness is a hyponym of pain; that is, unpleasantness is a word with a more specific, narrower meaning than pain:

pain: emotional distress; a fundamental feeling that people try to avoid
unpleasantness: the feeling caused by disagreeable stimuli; one pole of a continuum of states of feeling

Other hyponyms of pain include growing pains, mental anguish, hurt, suffering, distress, hurt, suffering.

pain (noun)	unpleasantness (noun)
a somatic sensation of acute discomfort	the quality of giving displeasure
something or someone that causes trouble; a source of unhappiness	the feeling caused by disagreeable stimuli; one pole of a continuum of states of feeling
emotional distress; a fundamental feeling that people try to avoid
a symptom of some physical hurt or disorder

pain (verb)	unpleasantness (verb)
cause bodily suffering to and make sick or indisposed
cause emotional anguish or make miserable