When upright and not wearing a bra (problem very positional), my right nipple turns purple. left unaffected. no pain beside regular menstrual cycle pain, no discharge. had ultrasound which came back normal. cannot feel any lumps. should i worry?

"darkened nipple" ?: Darkening of one side as opposed to both nipples may be related to a disparity in size between breasts, which is normal. and may result in the larger of your breasts having more dependent blood flow, which colors the nipple differently than the smaller breast. An ultrasound is not enough to investigate breast disease, so get a digital mammogram done to be more certain.