What you have is a condition called as urticaria. It is basically an increased skin sensitivity response . It happens because of various triggers and the trigger is most often different for different people. the line that you describe it happens after scratching is something called as physical urticaria and it is also a manifestation of demographism. A thorough Examination and detailed history evaluation would help to accurately make a diagnosis. For you can take tablet levocetrizine 5 mg to subside the itching you can visit me my clinic for further evaluation. Hope this was Helpful.