Mel B always had fans — plural. Now, at 50, the Spice Girl has a singular fan. That she carries with her everywhere. To manage her hot flashes. Before she learned it was menopause that was causing her sudden onset night sweats, brain fog, anxiety and vaginal dryness, the pop music royal (and her “quite a few years younger” husband) wondered if she might just be “losing her marbles.” “I’d be talking, I’d lose my train of thought,” Mel, whose full name is Melanie Janine Brown, tells The Post. “I’