Food allergies are actually not the most common cause of atopic dermatitis (itching).
Signs Your Pet May Have Allergies
Allergies can develop at any point in life, and unfortunately it is pretty difficult to figure out what is causing them. I think it would be helpful for you to record when this is happening and how severe the symptoms are you so you can see if it is related to something she ate, the weather, possible fleas, any changes in bedding or cleaning agents, and anything else you can think of. You may find this is not related to food at all.
Was the expensive dry food she was on eliminating her symptoms? If so, I would use that to conduct an "elimination diet," where you feed her that food until her symptoms go away, and then add in ingredients one by one to see if she has a reaction. It can take a long time for the symptoms to go away (my vet said eight weeks, I have heard anything from 6-16 weeks).
DIY Elimination Diet for Dog and Cat « Dr. Matt Allworth – Veterinarian
If you are going to make homemade dog food, you will need to do a lot of research to make sure you are feeding something that is complete and balanced. There is a lot of information on the internet but unfortunately a lot of it is bad information.
My dog has iffy allergies that are definitely environmental but might also be related to food. I tried a raw diet for a while and it helped...until the weather got warm! Then we were BOTH itching and rubbing our eyes (yay for living in an agricultural valley). So then I knew it wasn't the food after all, even though she does have a sensitive stomach and seems unable to tolerate certain ingredients. We are trying a fish and potato-based kibble and it seems to be helping so far. Raw was also great (her most recent blood test showed her to be in excellent health) but I have had trouble with her getting bad gastroenteritis from certain ingredients.
Something that can help with itchy skin is giving essential fatty acids to your dog to build up the lipid layer on their skin. These are found in marine fish oils and things like evening primrose oil, safflower oil, and coconut oil. I would use products meant for humans and ask your vet about dosages. You can find good stuff at natural food stores.