heartburning vs bitterness

heartburning and bitterness both are nouns.

Word	Noun	Adjective	Verb	Adverb
heartburning	Yes	No	No	No
bitterness	Yes	No	No	No

As nouns, bitterness is a hypernym of heartburning; that is, bitterness is a word with a broader meaning than heartburning:

Other hypernyms of heartburning include gall, rancor, rancour, resentment.

heartburning (noun)	bitterness (noun)
intense resentment	the property of having a harsh unpleasant taste
	a rough and bitter manner
	the taste experience when quinine or coffee is taken into the mouth
	a feeling of deep and bitter anger and ill-will