I don’t think scratching is causing your bumps.
In the case of “dermatographia,” the skin starts to itch, often caused by something as simple as a bra strap or belt rubbing, or heat or cold. The scratching itself is what raises the bumps.
“Signs and symptoms of dermatographia may include:
Raised red lines
Swelling
Inflammation
Hive-like welts
Itching
The signs and symptoms may occur within a few minutes of your skin being rubbed or scratched and usually disappear within 30 minutes. Source is my link above.
How do I know? I suffer from it. When I start to itch, I can draw a tic-tac-toe board on my thigh with a thumb nail. Within seconds, raised red itchy lines appear. Hence the name
The root cause or allergen is hard to figure out and requires detective work from the Dermatoligist. Usually it is treated causally…live with it, Benadryl or Zyrtec.