Hello,
It is not possible to provide any kind of helpful advice on a lump or a bump from a photo. This appears to be under the skin but even with that I can’t help direct you as to whether this is an acute swelling from an injury or insect bite, or the beginning of something more concerning like a cancer or infection.
The only way to help is to see your vet and let them examine it in person. From there they may recommend it be monitored or an aspiration or biopsy be done.
Hello,
It sounds like a more aggressive approach might be warranted. I would ask the vet to consider doing blood work, culture the wound to look for the best antibiotic and hospitalization with iv fluids.
That would be my plan if this were my patient.
Best of luck.
Krista