This is just one bump? If it were swollen or painful, I'd say see a doctor as you might have a little infection. Here's an article. https://www.healthline.com/health/skin/bumps-on-nipple My thought is that it is probably something to do with the hair follicle. We also have glands on our ariola that are called Montgomery glands that secrete oil. This could become clogged or infected. If it continues for a while and doesn't go down or if it becomes painful, hot to the touch or anything like that, I'd go see your doctor.