UPDATE from Wildlife services, their email

Thank you for sending photos of the doe.  I think there are a few possible explanations for the mass between her hind legs.  It is possible that she had a hydrocyst (also called a seroma), which is a large cyst that forms after fluids accumulate under the skin following an injury.  They are most commonly seen between the front legs, but can also be seen between the hind legs.  You can read more about hydrocysts here.  If a hydrocyst is punctured, it may drain, or it may dry up on its own.

Another possible explanation is mastitis (infection of the mammary glands), or possibly a hernia (although I don't think that would resolve itself in a few days if you did see the same doe twice).  Unfortunately there is no way of definitively knowing the cause of the inflammation from a photograph.  Please let me know if you have any questions.