What causes red rings on dog's stomach?

Well in most cases its not anything. I really wouldn't worry yourself. You should probably take he/she to the vet just to check it out. It could be a hernia. A lot of dogs have had hernia's. It doesn't really hurt them, it's just something they have. There is a possibility it is cancer, but that is a slim chance. Just have your vet check it out. I'm sure he/she is fine. Good Luck! :D