It sounds pretty clear that this is some kind of irritation problem with your bra. If it goes away completely with a better fitting bra, then I think you don't need to worry. However, if you think that the spot is always there and gets worse when it is irritated, then you might need to check with a doctor. This doesn't sound like a breast cancer symptom, but constant irritation of a mole or spot might cause it to be infected. Definitely call it to your doctor's attention at your next appointment.
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