Wondering if it picked up on some low level tension from the movie itself, like a scene that felt off even if you weren't fully stressed. Makes you question what "calm" really means.
Movies can mess with your nerves without you even realizing it. Stuff like creepy music or a weird shot can set off your stress response. So that ring might be spotting the anxiety your mind hasn't admitted yet.
Do you think our bodies pick up on subtle cues we ignore? @robertblack's point about movie tension fits a pattern I've seen. My smartwatch often alerts me during seemingly calm tasks, like reading emails. It's usually because I'm subconsciously worried about a deadline or something.