For reasons of it being easier to treat if caught sooner, there's no harm in having her checked for thyroid or pituitary gland imbalances, her peditrician or your family doctor can do it or send you to en endocrinologist. It may just be nothing, or maybe she just needs to stop consuming dairy products, but it's really easy to check just in case that's what it is! My 7 year old niece was diagnosed at age 6, they had her seen by the doctor because of the BO and it turned out to be pituitary.
I sincerely hope that all the other posters have at least mentioned this to their child's doctor, because the only six year old I've ever known with BO had a medical problem. I've done a lot of work with children over the years..babysitting, camp counseling, cheer coach, pageants, and knowing my daughter's friends, and have never encountered one single kid with BO at that age, other than my niece. If there is a hormone imbalance it can seriously mess with their growth and fertility. I do not think this is normal from my experience, and it surely doesn't hurt to run a simple blood test just to be sure!