While pregnancy is definitely the most common reason to not have your period when you are sexually active, it is not the only thing that can cause your period to stop or to be irregular. Missed periods can be caused by many things including low weight, hormone imbalances, some medicine and even stress. Hormone imbalances can also cause acne and extra hair growth. Make an appointment with your health care provider (HCP). Your HCP will likely order blood test(s) to help figure out why you are having irregular periods. It is unlikely that the color of your nipples is related to missed periods, but it’s important to have that checked by your HCP as well.