[header] How to diagnose lumps in guinea pigs [title] Look for abscesses on the head and neck. [step] Abscesses under the skin are very common in guinea pigs. They are the body's attempt to' wall off' an infection and keep it from spreading. 
Abscesses can form either after a bite from another guinea pig, or if something rough and sharp (e.g., straw) goes through the skin. They can form anywhere on a guinea pig's body, but the most common locations are the head and neck.