[prettylily]

It might actually end up being hormonal, especially if they occur more often during a particular part of your cycle. Sorry I can't be more help.

http://www.pregnancy-info.net/forums/signs_of_pregnancy/pimples_on_nipples_not_areola_/

When you pregnant you can get white pimple like things on your nipples, but hormones before your period can also make them look this way.