It could be a tumor although abscesses are more common. Abscesses can also develop in the skin or at the tooth root. Guinea pigs can also get other types of skin tumors--some of which are benign and cured surgically. Have your guinea pig examined by a veterinarian to determine what type of growth this is. Initially a fine needle aspirate may be needed--this procedure involves using a needle to collect some cells to examine under the microscope.
I am having the same issue--it is his third growth. They seem to easily pull off after 2 days. Did you ever get this resolved? I have been to the vet twice, they gave antibiotics, but it doesn't help.