I would have to think that is a tumor or cyst. There are many types of tumors in chickens, including the ones caused by Mareks or leukosis viruses. Since it grew very fast, I would lean more toward tumor. Unfortunately, about the best way to know what it is, is to get a necropsy by your state vet if she dies. They take tissue samples and do histology to help diagnose. In some states it can be easier and more inexpensive. You can call the state vet, and they can tell you how to bring in the body or can possibly send an overnight shipping label. Here is a link with most state vets to contact:
https://www.metzerfarms.com/PoultryLabs.cfm
https://www.metzerfarms.com/PoultryLabs.cfm
. She seems fine other than these spots. So I’m hoping she continues to eat and drink normally. 
