You might be able to start narrowing down the list, though, if you keep a journal of what
happens in your pet's daily life and how the itchiness
is on a day to day basis.
Get a small notebook that you will use ONLY for your pet's allergy diary. Each day, write down what the weather is like, where your pet has spent her time, what she has eaten (even small treats, everything counts here), and how much the itching has seemed to bother her that day.