Nipple discoloration?

Erin • Skyla IUD . Happily with my Army Man
Hey there so I just got out of the shower and when I looked in the mirror I noticed one of my nipples had turned a dark purple, like the color of a bruise. It went away after about five minutes. My normal nipple color is a light baby pink so I thought this was odd. Does anyone have an idea of what the discoloration was about?