If it's red and warm to the touch, that's a sign of infection, call your doctor. In fact call your doctor either way because with the exceptions of your breasts possibly growing and your nipples darkening, you want to alert them to any other physical changes.
When someone says "you look like you have bloodshot eyes" is what underneath my my left breast looks like. The right breast is similar but no where near as bad
I have the same thing under both my breasts. I'm not too worried about it. As long as it doesn't itch or isn't irritated, then I'm good. It appeared when my breasts started growing again, so I feel like it's stretchmark-related
Re: Breasts
Married 9/28/13
DS born 11/12/15
EDD 8/13/18
When in doubt, ask your doctor--not the internet.