Welcome to Yorkie Talk. I'm sorry that Penelope had this lump and diagnosis. I never knew that Benadryl would be prescribed for such a thing, but then I read this article explaining mast cells and histamines.
Why Benadryl For Mast Cell Tumors? - Dog Cancer Blog
There are some interesting comments on the article from people going through this with their dogs. Some of the comments may be helpful to you. Sounds like meeting with the oncologist first is the right plan of action.
Best wishes for Penelope. Keep us updated.