This is a normal erection. Learn more about the mechanism of "rising nipples" and what can cause their erection in men and women. External stimuli provoke contraction of muscle fibers:
Or a psychological state, illness.
The nipples swell and come into a state of increased tone as a result of external irritating factors, as well as internal reactions of the body. Until the end, this mechanism has not been properly studied, therefore today there is no unambiguous answer to this question. It is believed that the nipple "rises" as a result of irritation - this contributes to a more comfortable grip of the nipple by the baby when feeding, in addition, the process is also triggered by sexual arousal in men and women - apparently evolutionarily this sign of a partner's reaction to arousal was recognized by nature as an effective indicator partner's readiness for mating. In addition, this mechanism is also combined with ASMR arousal, which has nothing to do with sexual arousal. The same trigger is responsible for the appearance of goose bumps on the human body, which is often accompanied by pleasant sensations of a non-sexual nature. In animals, this mechanism raises the coat on end, which usually occurs in a state of stress and the triggering of the self-preservation instinct. In humans, this reflex produces a pleasant sensation called the autonomous sensory meridional response (ASMR).