Sometimes my left breast's (the meatier one) nipple turns a dark shade of purple while the other stays pink. They're never both purple although my left nipple isn't always darker. Usually if I massage it or my boob, it changes back... And forth.

Is this a sign of breast cancer or a blockage of some kind? ? ? ?

Note: I just recently saw my gyno for a pap smear and she did a breast exam on me but didn't feel anything I guess. Also I forgot to ask her this while I was there haha.