The female nipple is mainly composed of breast ducts, circular radiating smooth muscle and connective tissue. Since the nipple is a sensitive organ with nerve endings, it is normal for the smooth muscles around the nipple to contract when stimulated locally or sexually, causing the nipple to erect and stand up. In addition, when there is local inflammation of the nipple, the nipple is red, swollen, painful and also appears to stand up. This is caused by a local blockage in the ducts of the nipple and bacterial invasion and infection. Nipple edema can also occur when the nipple is affected by trauma such as impact and compression. These are the more common causes of nipple stand-up, most of which are due to stimulation of erection. If there is local pain, redness and swelling, you need to go to the hospital for examination to rule out disease factors.