What Causes Itching On Arms And Neck?

my skin itching started 3 days ago my arms, and my neck, but worse of all is my back, mostly the left side I have had that itch for about 2 years and is getting worse and has cause to have scabs from all the scratching. I think that the new itch is something I ate. I have taking Benadryl two times a day it has helped. your assistance regarding my back would be appreciated. Would a blood test show up my new itching on my arms and neck sometimes it feels like aunts crawling. Thank you much