Thanks a lot to all of you for advice! I also was afraid that this is ringworms and started a treatment with lotrimin. However, today, I was able to show my guinea pig to a veterinary after the Christmas holidays. He told me that it's mites. He put 2drops on her and told me to come back. It's hard to tell from one small picture. But I really appreciate your time, and, once again, thank you for advise.