I wonder if that sore could be some type of bite like a spider bite that could be causing these symptoms. Are you certain it is from a cactus? Was there any type of insecticide that could have been sprayed on the cactus that might have been on the cactus spike if you are certain? How long ago did she make the move from the field to the city? Is it possible she is experience some type of depression and has not acclimated to her new environment yet? Dogs can be pretty sensitive to changes like that, especially if it is a major change like this one. If her symptoms do not improve I would take her for a checkup at the vet just to make sure everything is alright.