Allergy test

I think my two month old maybe allergic to dogs. We don't have a dog at my place but my parents do and when she goes over there around the dogs she starts to get rashes on her face. We haven't been over there or around dogs in about 3 days and her face is clear now? Is that we could cause it? Her dad and half sister are both allergic to them if that helps? We don't go in for her doctors apt till next week and just wanted to see other moms comments on this situation.. How old do you need to be to get an allergy test just to make sure?