Cat has a soft lump under the neck skin, slightly red around it, eating and drinking fine, good spirits

A cat’s subcutaneous soft lump may be one of several conditions, so a prompt veterinary check and treatment is advised. First, a subcutaneous hematoma feels soft with little pain and a fluid wave, and usually absorbs slowly; if clotting is impaired, hemostatic drugs are needed.

Second, inflammation from an injection site, usually treated by puncturing to release fluid along with anti-inflammatories. Third, local cellulitis with a pus focus, which needs systemic antibiotics and wound debridement if body temperature rises.