You should show it to vet immediately. There can be cleanliness issue or an allergy over there but there is a possibility of cancerous growth as well. It is better that you do not put your puppy on any antibiotic before having a proper test like Biopsy. Such a small pet is very vulnerable to skin diseases and needs extra care. You should also take care of its mother's personal hygiene. Try to clean the area gently with cotton or some soft piece of cloth. If it is removed then well and good, if it bleeds then alarming, and if stays there without bothering your puppy in any way then wait for a while. There is a chance that it automatically heals if its immune system is strong in case of healthy puppy