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.