Since few weeks i am having itching anywhere on the body and upon scratching there is rash which stays for half an hour or so. what could be problem?

Dermatographism: Sounds like dermatographism. It's a type of hives ( urticaria) and merits a derm evaluation if persists. Try some Allegra (fexofenadine) to control it.

Dermatographism: Dematographism is the ability to cause hives by scratching or stroking the skin. Dermatographism is especially seen in individuals who have an itch or hive problem to begin with but could also occur without underlying hives or itching.