you didn't say how old she was, did she just get spayed? if so its a stitch that is infected. if she is a pup , it is likely a umbilical cord infection. not a hernia---do not worry it is likely not to bad.until yo can get to the vet , put a warm as can be cloth on the belly 3 x a day that has been soaked in Epsom salt and water. do not panic.and let me know .between feeding a great food, daily face, feet, private area cleaning. and keeping flowers out of the yard that the dog plays in , and giving my own vaccine booster shots!! I see the vet very rarely !!! my vet helps me keep the visits down by also filling as many RX scripts , at my drug store.