What Does Dark Line Under Each Breast Indicate?

I have a dark line under each breast, right below the crease. I'm 25 years old, never been pregnant, and a 40D. I wear a correctly-fitting bra with underwire. The lines appeared a few months ago. I am not on any new prescription medications, but I was recently diagnosed with dermagraphic uticaria and have been trying various antihistamines off and on for the past 3 months. The medications I do take are lexapro, seasonique, and applenzin. What is causing the dark lines and how can I get rid of them?