Yes a cancer can open up and look raw and drain. Typically though it does not heal it stays open and sore looking.
It may be that what happened is she had a clogged milk duct that got infected and then abscessed to the outside of her skin.
It may be that during mating the male cat punctured her skin with a claw there and she got an infection from that pushing bactieria under the skin. The antibiotics likely helped it concentrate and now that it has ruptured and drained it sounds like it is healing from the inside out.
You can clean with Betadine iodine solution or the peroxide and that won't hurt the kittens.
You can try a small dab of Neosporin ointment on the spot too which should not bother the kittens should they lick it.
Before doing the above you can use a warm damp compress on the spot to help draw infection and increase circulation to the area to promote healing.
You may want to ask your vet for another course of antibiotics that will be safe to use in a nursing mother.
Hope this helps you!
SHE DID HAVE A THICK CHEESY CRUST TYPE STUFF, OF OLD YELLOW LOOKING DISCHARGE, FROM THE NIPPLES ON BOTH SIDES OF HER WOUND WHEN I TOOK HER TO THE VET BEFORE IT OPENED UP. SO I THINK YOU'RE RIGHT ON POINT. THEY'RE PROBABLY RELATED. THE MALE DOESN'T HAVE CLAWS BUT I BET SHE PROBABLY GOT IT FROM A KITTEN FROM HER PREVIOUS LITTER. I DIDN'T WANT MORE BABIES BUT THEY DIDN'T TELL US SHE WAS PREGNANT. BY WARM COMPRESS DO YOU MEAN JUST HOLD A WARM CLOTH OVER IT? WILL BATHING HER HELP OR DO YOU THINK I SHOULD WRAP GAUZE AROUND IT TO PREVENT MORE INFECTION FROM THE KITTENS LIL CLAWS?
A warm compress is usually something like a cloth dipped in warm/hot water, wrung out, and held against an infected area to warm the spot to increase blood flow to the area. That can speed healing with the added circulation to take away debris from infection and bring in white blood cells to fight it, and tends to draw infection out into one smaller area from where it was spreading.
I would not wrap it but keep it treated and let it drain. Draining lets the infected material leave the body as the body heals instead of trapping it in healthy tissue.
I usually start trimming nails pretty young so the babies don't scratch mom as they nurse.
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Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care
You are very welcome - hope mom and kittens do well!
Thanks for the above accept!