Thank you for your responses.
Hormones, especially during or after pregnancy, can do many things! Progesterone and estrogen, among other things, stimulate melanin production by the skin. This can lead to hyperpigmentation of the skin in certain areas. These hyperpigmentations are brown in color and not raised, which is what you describe.
Since we are not sure when the spot appeared, it makes the circumstances of onset a little more unclear. However, since it's a spot that's never exposed to the sun, it's much less concerning. π
So for now, I'm not worried about it. I would still recommend that you keep an eye on the progress of the stain. If you see it change in appearance (shape, color, elevation, size, etc), I recommend you see the doctor for a physical exam.
I hope this gives you some guidance. Please feel free if I can help you further π
Veronique Leclerc, pharmacy intern