What Causes Chronic Itching On All Over The Body?

for about 4 weeks now I have been having cronic itching episods starting on my should and moving around my upper body, back, stomach. I have now started with the same itiching on my thighs however after scratching have noticed little bruises in some places and a large (approx 2 inch) bruise on the inside of my left thigh which is dark red/purple, any ideas to cause