Usually, the kitten teething period starts at between 3 to 6 months of age, and is completed between 7 to 9 months of age. Bad breath is normal during the teething stage. Kittens often suffer fever and/or diarrhea during the teething stage, and as long as your kitten is eating and drinking, she should make it through the teething stage just fine.
Your kitten may also be more lethargic than normal while she is teething, especially if she has a fever. Take her to the vet, however, if she stops eating or drinking, or if she acts like she is in severe pain.
I hope this helps to put your mind at ease --- both of my cats had difficult teething stages, and today they are very big and healthy! Please let me know if you have more questions.