What Causes Body Itching After Smoking Cannabis?

Hi, i was wondering if smoking canabis can have an adverse affect on the body with regard to itching. It happens all over my body randomly and not neccesarily in the same space, sometimes it can fell like a minor nettle sting or even a bite. 'i know what your probually thinking' :) I have had several visits to the docs and each one has failed to find anything as there are no rashes or visible signs of any nature. I am trying to wittle down the cause of this though it is becomming a real.....frustration shall we say? Hope you can help, cheers clem.