Did you mean that you get a welt
by scratching a bit or if you the welt
was very irritative?
This may be an allergy of some sort and
you may have to go to the doctor to find out if there is something you can do to avoid contact or allergy due to the inducer.
Meanwhile, you may use some anti-allergics, keep the skin moist and
calamine lotion over the skin.
Let us know what the doctor says and
if you have any other questions.