Your nipples are probably not "sucked in", but simply inverted. Some people's nipples are just naturally this way, and is not harmful or bad in any way. Everyone's breasts and nipples vary wildly, despite what you see in the media.
Inverted nipples are not normally an issue for breastfeeding or anything else. Surgery is possible to change the shape and protrusion.
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This nipple erection is caused by an increase of blood to the area usually taking place when physically or mentally stimulated or reacting to cold temperature, the change is not permanent and nothing to be concerned about as it happens to all women and even men at some point or another.