What Causes Itchy Rashes On The Body?

I ma 48 years Old and I am having Sugar and blood pressure under control, Last one month I developed Itching in my lower part of Leg Below the knee and slowly it became black patch and same thing developed in both leg ..now in few days back i developed so many pimples on my left hand and some on right hand and some different part of body too..and feel itching Feelings there applied Candidera Tube and took Tablets Centazine for 20 days and Conflu150 once in Week for 4 weeks, but i got no relief..inspite of that it keeps on increasing.....please advice what should i do.. ?