First of all it can be due to hormonal changes. Progesterone and estrogen cause the body to produce more pigment. This is why many women see an even change, splotches and patches of darkened skin all over their body. Other causes of darkened areolas in women can be due to pregnancy, aging, menstruation and medications. It can be due to menstruation in your case.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Mine are doing the same thing. I just turned 30 on the 5th of Feb. I'm on the 2nd day of my period so I know I'm not pregnant as well. I fairly positive it's not menstrual related as I noticed it starting a few weeks ago. My nipples are getting darker in splotches, not evenly. I'm also noticing dry skin there as well now too. Could it be hormonal? My mother and oldest sister went began going through menopausal hormone changes in their late thirties... Could I be starting to see early signs of this now?