I'm really sorry to hear about your cat. It's heartening to know that he was able to recover from his initial injuries, but this new issue definitely sounds worrisome. Although it's a good sign that he doesn't seem to be in pain and is acting normally, any unusual changes in your pet's physical condition should be checked by a professional.
Please consult with a veterinarian as soon as you can. They have the necessary training and expertise to determine what this protrusion could be and whether it's a cause for concern. This could be related to his previous injuries, or it might be something completely different.
Remember, it's always better to err on the side of caution when it comes to the health of our pets. Keep a close eye on his behavior for any changes and try to arrange a vet appointment as soon as possible. Please keep us updated on his condition. Wishing you and your furry friend the best!