u will have to ask dr.
has she had menses
r her breasts budding
these things have become more and more in our Industrial and sophisticated societies and these in nine yr olds compared to 150 yrs ago menses started at 16 yrs. That is a change that seems impossible on a evolutionary scale.
Hopefully it is just a pimple and not a boil which would be far worse. It may just be a cyst or cellulitis but check in the dr if possible or at least call dr office and ask the nurse.
When I was about that age (maybe a year or 2 older) I had the same thing come up. I hadn't started menstruating yet. My mother took me to the doctor to have it checked on and they said it was a "breast bud" or the beginnings of the formation of mammary glands. I believe it is common, even if it just starts on one side. You can always take your daughter to the doctor to make sure, it never hurts to know for certain.
How about checking to the left and to the right of her spine between her shoulder blades. Use a finger and push every bit of areas without missing a space from the bottom of her shoulder blade to the top. Do both sides and make sure she is on her belly and in bed to do this and rub some baby oil first on each side of her spine. See if she has any pain in those areas. If she does, this mean that she has some pinched nerves which need to be reset soonest. I have had 2 young girls, age 10 and 11 who had the same symptoms/problems. I know as I have been a healer for 14+ years.