The problem with dermatographism is that most of the times there is no specific cause for it. However at times hot baths, stress
, exercise and skin friction( rubbing, scratching, minor pressure over skin) can flare it up. The disorder usually goes into remission on its own , may be in few days or it might take few years to go. So the only thing that can be done is to take antiallergics to relieve the symptoms. As you said with allegra
you are not having much of response, in that case you can consult the doctor to add one more group of antihistaminic. You can also apply soothing calamine
lotion, it will reduce the itch to some extent. Take care.