I'm A 15 Year Old Girl. I'm On Birth Control

I'm a 15 year old girl. I'm on birth control but every once in a while I forget to take it. About a month ago I had unprotected sex with a guy but he pulled out before anything could happen. I then had vaginal discharge that was a brown color but then my period came three days early. Today I was in the shower and noticed that my nipples were a dark purple. What does this mean?