Can nipples darken due to something other than pregnancy?

I've noticed over the past few months my left nipple has been turning this purple color when I'm up and moving around. When I lay down or am just chillin it's a dark pinkish color again.

I'm not sure if its always been like this because I realize I've never quite looked at my nipples before now.

I have the Nexplanon implant and was on combo pills about 5 months ago. What could be causing this?

I'm definitely not pregnant according to several hpt's and a blood test.