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Since you don't know what caused it, you should clean it well with peroxide and keep it dry.
If you don't see it start to improve, you will need to see the vet.
I would be most concerned that it is mastitis, a breast infectio that can rupture the breast.
If you see lethargy, vomiting or disorientation and not eating, you need to see the vet sooner.
Let me know if you want to discuss this more.