That sounds a bit odd to me. Did the breeder tell you she was good until 6 months? If that is the case maybe the breeder only gave her 1 or 2 sets and told you to get her some more at 6 months...which is not how things are supposed to be done. Usually puppies get 3 - 4 sets of shots from the age of 6/8 weeks up until 16 weeks. After that shots should be good for around a year.
Also I have heard that rotties are just a breed that seems to be more prone to this awful disease
. Luckily she isn't an infant puppy anymore and she should be strong enough to pull through this