Tib,
It's "erect" not errected (not meant as a spelling flame).
And in my in-expert opinion: All skin shrinks when it gets cold, partly
due to the shrinkage of capillaries to maintain internal body heat.
Some parts of the skin (due to thicker surface skin in those areas, or
the body structures they cover) can get stiffer under those conditions.
Most nipples tend to go outward by design, so are more likely to
do so under shrinkage as well... Most women (and some men) also have
enough muscle and/or fat in the breast to create a support for the
stiffening nipple, making it more noticable when pressed against fabric
than it might otherwise have been.
This also helps to explain why nipples can get erect when they are
not cold. In that case there is a reverse expansion caused by increased
blood flow in certain parts of the body caused by exersize, excitement,
and arousal.