What Causes Severe Itching All Over The Body Despite Taking Pinworm Medication?

I am a 28 year old tall, slim, male who smokes on a daily basis. About 5 months ago, my anus area began to itch so badly that I would scratch and make it bleed. I went to the doctors they gave me medication for pinworms and then told me to stop drinking caffeine, which I did. The itching has now spread over my whole body, to the point I am constantly itching myself and now causing myself to bleed. I cannot fall aleep at night and all I think about during the day is itching myself. After taking the pinworm medication and being caffeine free for 5 months, the itching near my anus has not went away either. Please help, this is destroying my life.