Has the dog been outside?
Are there any wood piles near by?
Didi you notice any little marks where the lump is? Joan Jav917
I am concerned about your description, because of the dog being outside this sounds like a Spider bite and allergic reaction. The most common bite at this time of the year is a brown recluse spider. The problem is they are very toxic and cause necroses of the tissues where the bite occurred. This is a situation where the dog should see a Vet asap. The Vet may have to give some cortical steroids, Antibiotics and fluids. This is not a wait and see situation. You have done everything you can to keep the toxin from spreading, but unfortunately the Vet. has better facilities to mange this type of situation.
I am including a link about Brown recluse spiders and a Warning the picture are graphic www.highway60.com/mark/brs/bite.asp?msg=1779 Joan Jav917
Since you have been treating with Amoxicillin and Benedryl this may have help out the problems from getting worse. This could also be an allergic reation to the Amoxicillin. The biggest thing that worries me is that the swelling has increased and the areas are getting purple, which can indicate some severe infection.Joan Jav917
Please post an update after you have seen the Vet. I would like yo know what the Vet's findings are. I appreciate the accept and would appreciate positive feedback when you have time. I will watch for the update on your baby. Joan Jav917