Um , weird. MY dog Lola had puppies a few months ago and her nipples did the same thing. Maybe she is just hot? Has she had a bath recently? it could be anthing, Lice , Fleas etc. Or she could just have a rash.
Have you looked at it closely to see if there is possibly a bite or some irritation or lump? Is there any discharge from the nipple or do you see pus? Sounds like some kind of infection going on, especially if it feels hot. Mastitis woukd likely involve the whole nipple. Do you have a "rectal" thermometer and know how to take a dog's temp?
See if she will tolerate you gently washing it with warm water then try to apply warm compresses.
If this is an infection, it really should be checked by a Vet so they can determine the cause as she may need to be on antibiotics.
Don't let that get worse and please come back to update.
Has she been spayed? And when? If it was before her first heat cycle when she was a pup, then she will be 90% or more protected against breast cancer. If not...then that IS a possibility. Especially if she is middle-aged or older.
A vet needs to look at this.
Please do not think canine breast cancer is automatically a death-sentence because it doesn't have to be. Many mammary cancers are completely curable if removed in the very early stages.