Queeny, thanks for using our service.
Anytime an unidentified growth has started in the skin, it needs to be checked out by a vet. This could be something as small as an abscess or as major as cancer. Because it's broken through the skin, your baby needs to be on antibiotics. Depending on how severe your vet feels the condition is, he may suggest sedating your companion and either removing the growth or debriding the area. If its removed and thought to be cancer, histopathology to determine exactly what the growth is can help regarding prognosis and treatment.
If this area is not currently bleeding a fair amount you could probably wait to have your pet seen on Monday. You can keep the area clean until Monday by gently washing it twice a day with antibacterial soap and water. I would recommend seeking emergency care for your companion if he stops eating, drinking and feeling normal in your care at home.
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Veterinary Nurse
8 yrs experience with animals, 3 in emergency med. Owner of 3 APBT and 3 sphynx