The first lines of "Medusa" immerse the reader in a suffering, jealous mind.
The speaker begins with a dramatic, ominous tricolon: "A suspicion, a doubt, a jealousy." The asyndeton here, which puts these words in sequence without a conjunction like "and," means that it's not clear whether these three words describe separate threads in the speaker's thoughts, or whether they are just three ways of describing of a single emotion.
This ambiguity unbalances readers right from the start, and leaves them unsure what's going on in this suspicious, doubting, jealous speaker's mind. Does she herself even know whether these feelings are real, or how they connect to each other? The quick movement between these words suggests that she's trapped in swift, painful feelings.
Her use of the word "grew" also feels rather sinister. Her feelings seem to have their own nasty life, and they're only getting bigger.
This introduction, with its pain, confusion, and urgency, prepares the reader for a new take on the Medusa of the poem's title. This will be a poem that pits misogynistic tropes about women against a more complicated and sympathetic portrayal of a famous female monster.