It's probably her giving off "the vibe". Pheromones are real. Not that it's guaranteed she wants to act on it. Otherwise you'd want to fuck every half attractive you meet.
Every once in a while I meet a woman who gives off that vibe. I always wondered if it was just me being turned on by a sexy woman, or her giving off the horny vibe, or both. I hold back, not wanting to give off that perverted impression. But, wow, my cock yearns to share in some fun times.