I think it is worth having this checked out by your doctor
. I can think of some normal causes of the skin of the areolas getting lighter or darker - for example with pregnancy the areolas tend to get somewhat darker. If you were recently pregnant, they could be getting lighter now again now that the pregnancy hormones are gone.
Otherwise, I would be concerned to exclude infections of the skin that are associated with skin lightening, especially fungal infections such as tinea versicolor. It would be unusual to have a fungal infection just on the areolas, but not entirely impossible. Your doctor should be able to sort this out quickly with a physical examination.
Another condition that needs to be ruled out is vitiligo. This is an autoimmune condition of the skin in whic the pigment cells within the skin are targeted. The result is slowly enlarging patches of skin without pigment. Generally speaking, vitiligo results in markedly white patches of skin, which isn't exactly what you are describing; nevertheless, because the condition is cosmetically quite important, your doctor should rule it out.
I would start by going to your primary care doctor
. If they are not sure what is going on, they can always refer you to a dermatologist