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Hello, this is Dr. Jess. I am sorry to hear about your kitty! Do you happen to know what type of mass the pathologist thought your cat had?
Is the mass increased in size at all? Does it feel warm to the touch? Is there any pain in the area or discharge?
Ok. It is possible your kitty could have gotten into a wrestling match with the other kitty and gotten a scratch, or sometimes masses that are cystic can drain. This isn't usually a concern unless they become infected or it occurs repeatedly. I would recommend keeping a close eye on the area until it heals completely. Check it at least daily for signs of redness, swelling, discharge, or heat. If any of these occur, an exam is warranted. You can apply topical triple antibiotic ointment (like neosporin) twice daily to the area.
If it will ease your fears, then there is absolutely no harm in taking her to the vet! They can be sure there isn't any sign of infection and that an antibiotic is not needed. It never hurts to have your kitty examined, so if your mind will be more at ease after she has been looked at, by all means go ahead and make an appointment for her!
I don't think the scab being red is definitely a bad sign. It could just be a new scab that hasn't gotten thicker or darkened yet. But it sounds like your mind would be put more at ease if your vet examined her. I don't think it is an emergency if she is otherwise doing well, but having an exam is never a bad idea. In the meantime, until you are able to have her seen or while you are waiting, I would apply triple antibiotic ointment and watch closely for redness of the mass (around the scab), discharge, heat, or swelling.