Poor baby, it is always complicated with skin issues. Could be so many things, but if you are rather sure she has not had fleas it is most likely not flea derm. could be contact derm. but doesnt really sound like that either. And the food, if there is an allergy, most probably an allergy towards the protein in the food, that usually dont do the tail, you see that commonly between pads and toes to start with. I guess it could be that you didnt rinse enough, mind you most dogs if the shampoo isnt rinsed off properly, would have problems sooner than a month after the bath. So a little puzzled here.
One thing that comes to mind, is thyroid. Where on the tail is the scab and have you had a thyroid panel drawn on her ever??
Does it look like it itches? if so you could always as a first aid get an anti itch spray and spray with, but if it is so she snappes after you, it sounds like she is really bothered by it, I would probably make an app with my vet. or at least call them and discuss the issue and see what they have to say.
Good Luck and give poor Zoey a kiss from me and keep us posted
Marit