I have these brownish tanish spots under my breast. what could this be? i was told that it could be the bra, from being too tight .

Dark pressure line?: Is it in spots or line where the pressure line is in crops?K itchy? Does it bleed when pick.

Tinea corporis: At your young age, i would doubt that you have worn a bra long enough or tight enough to cause hyperpigmentation. Especially if these are well-demarcated spots, and especially if they are itchy, i would lean more toward the possibility of ringworm. This area where you have warm, dark, moist skin-on-skin you could get a fungal infection. Try treating with Lamisil (terbinafine) or lotrimin.