weird question about boobs and bras

A little background so I'm 16 and no stranger to either boob or bras but, today in the shower I noticed some weird discoloration. Like right where the lip(?) of the bra stops (or like the part that try's to sneak out of your undershirt) there was a weird purple line following that on both of my breasts. I noticed it in the shower almost 30 minutes ago and it seems to be fading. Also it's not like I have big breasts or anything, generally I'm a 36c and the bra I wore to day has a little wiggle room in the cup department. So, I'm just confused to what it was and was wondering if anyone had something similar happen to them. Thanks y'all!