What Causes Severe Itching Leading To Blisters?

Hi, may I answer your health queries right now ? Please type your query here... For the last few months I have had severe itchy skin. It doesn't look like anything is there but after I itch I get small almost blister like bumps where I have itched. My worst areas are on my upper legs, behind my knees, in between my fingers and toes. I am actually bruising from the constant scratching. It's worse after a hot shower. My symptoms appeared after I started working in a long term care facility. I have tried cortisone creme ( otc) benadryl and mediated lotion. Nothing helps. Thank you for your time.