Brown patch in between breasts

I have this patch of brown in between my breasts (see pic; although I think it doesn't show that great) my older sister has the same thing; and when she noticed mine; she became afraid we have something.  She said hers has gotten worse; that it used to look like mine but it much darker and bigger now.. so naturally I fear it'll happen to me.  I think I first noticed it a few years ago. like 2.
what can this be?

[munshkin12]

omg. im 13 years old turning 14 in a few months and for a very long time a couple years maybe 3 i have had the same exact problem. At first i ignored it. then i through it was because im over weight. But not long ago i went to my stupid Dr. who told me it could psoibly be because of diabeaties but he didnt explain anything. So i tryed eating better and it sort of started to go away but then it came back and worse. then started to spread to my neck and behind my ears and my belly button and even in the front of my elbow. I really need help  i have been looking fro someone else who has this for so long so i dont feel so alone. I really need some answers. Is there anyone out there who can help meee?

[adashofblessings]

Ladies I have it as well. Munshkin12 you shocked me. I also have the spotting between my breasts, was told by a doctor to use selsan blue (sp?). They said to wash more. It was dirty spots and I needed to bathe more. It has now become such an issue that I scrub harder, even though I know its not going to help. I even tried a foot stone. Then I thought, hellooooo um, then wouldn't the rest of my skin on my body have issues. It is not because were dirty. I also have it on my neck, behind my ears and my elbows and knees are much darker. It feels like thicker, coarser skin. Lotion does not help me much. It is embarrassing to wear swim suits, v-necks, low neck line clothing,or your hair up. I also have looking for others with these symptoms. I do think it has a lot to do with weight, diet and such. I do think there is a link with these symptoms and diabetes. I don't know for sure, but the doctors I have seen, dont seem to know either.

[hmunday]

Go visit a dermatologist about it, it is most likely just excessive sweating that's causing a fungus to grow... mine was darker in the summer when it's really hot and humid. It has nothing to do with diet, but it affects big-breasted women or excessive sweaters. It's true though that weight makes it worse, it occurs in folds of the skin that moisture gets trapped in.

[anongurl]

SO HAS ANYONE FOUND A TREATMENT????