[Help] What can I do to help my dog stop her constant itching?

I have a year and a half old Aussie. Within the last month or so she's developed a really bad itching habit. Took her to the vet, who advised me that it's an environmental allergy and prescribed an antibiotic that has done nothing. He also suggested Benadryl... Which has also done nothing. She's itched herself to the point where there are bald patches on her back legs as well as welts/hot spots on her back where the hair is thinner.

I've tried anti itch baths, anti itch sprays, calming sprays... I'm not sure what else to try.