It was very shortlived
That sounds like Guglielma of Bohemia, a possibly-noblewoman-possibly-mystic from the 13th century associated with Milan. She’s an enigmatic figure, but her cult and the idea of her being the incarnation of divinity is almost certainly something that was propagated after her death.
We know that Guglielma and her son showed up in Milan in 1260, where she started living as a pinzochera, a religious woman living who lived fairly independently. They’re similar to the beguines of northern Europe. In Milan, she got involved with a tight-knit circle of religiously-minded elites and Umiliati. Apparently, she practiced simple, charismatic teaching and acted as a healer. She grew popular to the point of being the center of this little group, and when she died, those close to her started revering her in the usual way when you wanted to get someone sainted in the middle ages—saying she was a paragon of virtue, she denied being divine while alive even though her followers wanted to say so, etc. She was buried at Chiaravalle near Milan.
Things get more interesting after her death, because somewhere between her old age and her posthumous reverence, her following got the attention of the inquisitions of 1300. Over 36 Milanese elites were identified as revering her as the Holy Spirit incarnate, including a devoted servant girl who held a high place in the cult; one Andrea Saramita, a wealthy and almost obsessive follower who assumed the title of her ‘only son’; and an Umiliata named Sister Maifreda who acted like her vicar. According to testimonies—mostly extracted from Andrea—the cult saw Guglielma as a Christ-like figure who would return from the dead one day, equal in status to the Holy Spirit, and as a cover story, they carried on what looked like devotions to “St. Catherine” (nudge nudge wink wink). And if that weren't enough intrigue, she herself supposedly claimed to be a daughter of the king of Bohemia (thus, "Guglielma of Bohemia"). The testimonies claimed they believed in the priestly capabilities of women, salvations for Saracens, Jews, and Pagans, and of course, Guglielma’s eventual resurrection. The inquisitions deemed all this VERY heretical. Andrea and Sister Maifreda were summarily executed, and Guglielma’s bones were dug up and burned.
It’s worth noting that the records of the cult we have left come (as far as I know) exclusively from inquisition trials, meaning they aren’t the words of the cult members, but the words of the inquisitors deciding what to write down based on what suspects were saying, often under torture. It’s also true that this cult in particular was bound up tightly with Milanese politics. Sister Maifreda was the first cousin of Matteo Visconti, lord of Milan, and the lead inquisitor Tommaso da Como had some trouble investigating the cult because of how many upper-crust Milanese were involved, meaning the city in general resented his investigation.
…but there’s one more odd loose end to the cult. 150 years after the inquisitions and the burning, someone commissioned a painting of Guglielma herself in a little church in an insular mountain village called Brunate, not far from Milan, which still stands to this day. We don’t know what prompted someone in the fifteenth century to commission an icon of a condemned heretic, except that it probably escaped notice because Brunate is a very…out-of-the-way kind of village. We can't say that it means her cult persisted, only that someone knew who she was and held some kind of attachment.
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