When used without a qualifying prepositional phrase, a spell can be an old-fashioned term for a short period of diminished mental capacity. It can be used to mean a "spell of dizziness", a "spell of fainting", a "spell of lunacy", or even a "spell of epilepsy" (which would later take on the term "fit" rather than "spell"). The term is rarely used today without qualifying the specific type of spell the person is experiencing, but in olden times anything affecting one's brain was mysterious and hard to differentiate.
I can't find an online dictionary listing that old-time meaning, but the Google Ngram showing usage of the phrase "subject to spells" is interesting, and not only shows the declining usage over time, but searching the more recent works shows the phrase is essentially always qualified in modern times ("subject to spells of madness", etc) with the unusual exception of an income tax guide from 2000 (which likely is merely repeating the text of an earlier tax ruling or law). The phrase as a standalone seems to have died out in the 60's and 70's (see this example).