Often caused by chafing, friction caused by the rubbing of the nipples against the shirt while running. They’re more common in men and during cold weather (one of many advantages of wearing the sports bra). Can be remedied by covering with Body Glide, adhesive bandages or nipple guards, which are sold in many specialty running stores (duct tape works, too, but that's a more permanent solution and not really recommended). A.K.A., red elevens.
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