indigestion vs upset stomach

indigestion and upset stomach both are nouns.

Word	Noun	Adjective	Verb	Adverb
indigestion	Yes	No	No	No
upset stomach	Yes	No	No	No

As nouns, indigestion and upset stomach are synonyms defined as:

indigestion and upset stomach: a disorder of digestive function characterized by discomfort or heartburn or nausea

Other synonyms of indigestion include dyspepsia, stomach upset.

indigestion (noun)	upset stomach (noun)
a disorder of digestive function characterized by discomfort or heartburn or nausea	a disorder of digestive function characterized by discomfort or heartburn or nausea