Make sure you're not using "unscented" products, but "fragrance free". An unscented product can have a fragrance added to it for the purpose of masking the fragrance.
We've had good results with Salcura products (available online at their website - this isn't a promotion, I don't sell it or anything, it's just we've tried 7 million things for all kinds of itching for my dd and Salcura has been helpful where others haven't).
Also, consult an allergist. A dermatologist is a good start, but my daughter was immediately referred by a dermatologist to an allergist. You can try eliminating certain foods like dairy, or wheat, or corn, but only do one at a time, with a nutritionist's guidance. Allergies don't always manifest themselves in hives or swelling. They can produce itching, and some areas are just more sensitive than others. Turns out my daughter is allergic to something that causes eczema and itching, which we never would have guessed (a rare allergy).
You mention that she seems to have a habit of itching or scratching. Is she using a lot of hand sanitizers? They can be very harsh, and if she uses hand sanitizers with alcohol, which are very drying, and then scratches her bottom, she can be experiencing very dry skin there. Lose the hand sanitizers. Just have her wash her hands with a non-anti-bacterial soap and water.
Make sure she isn't consuming artificial sweeteners or lots of sugary foods. Clean whole non-processed foods only.
Good luck. We've been fighting the itching battle (only it's all over) for a long time. It's frustrating, I know.