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The initial lump is caused by the fluid in the vaccine. It is absorbed by the body within a few hours, days at the most. I think that you are dealing with either a vaccine reaction or other problem.
This is a common place for sarcomas so you will want to get this checked as soon as you can.
Let me know if you still have questions.
If you have more specific questions, feel free to ask. I would be happy to answer more questions if you have them.
I do hope that you take the lump seriously. Lumps in this area can be sarcomas that are induced by the vaccine. If that's the case, it's very serious and you do need a vet.
Let me know if you have more specific questions about this.