I'm sure there are a number of possibilities, but the two that immediately spring to mind are ringworm and impetigo.
For your sake I seriously hope it's not impetigo, as it is a highly contagious and truly disgusting disease. Despite the name, ringworm is not a parasitic worm, but a fungal infection.
impetigo
One possible cause is cigarettes. Seriously, I suggest researching the WebMD website. "Melanoma", a form of skincancer, is evidenced by black-colored sores on the skin.
Could be ringworm, a fungal infection in which the 'sores' are a rash in the form of a circle, red inside, and whitish-grey around the ring. Some people have said that ringworm sores look like cigarette burns.
Did you have an outdoor job? Have you spent a lot of time in the sun? If so, you had better see a doctor about those painful sores so he can rule out melanoma.
it is probably herpes...you should get it checked out
Friction burns/marks/sores caused by the nipples being rubbed by cloth of the material being wornm over them (Like a t shirt etc.)
Soap irritation, yeast infection, or possibly, inherited herpes
You can take the chance if you like, but to be safe, don't do it. The virus that causes cold sores is very contagious when the sores are in evidence and for a period before they show. Best bet is to be cautious and don't chance spreading this painful affliction any more than you have to.
the real cause of heart burn is acid in the tummy and burns like the mad
Cold sores look like a blister.
Sounds like a flea bed spot bathe her with flea shampoo and treat with neosporin if she can`t reach to lick it off
cheilosis: crack like sores at the corner of the mouth. (lip disease)