I have been experiencing the same thing! I have had hives that move all over my body for a month now. They started one week after receiving the Moderna COVID booster shot. I went to an allergy doctor who let me know it is called dermatographia. He informed me that is not that we are allergic to the booster but that we likely had the gene or whatever for dermatographia already and it was the booster that caused it to appear. He said they currently understand what is happening at the cellular level but do not understand why it is happening from the booster. He told me to just keep taking the over the counter antihistamines (like Claritin or Xyzal) because those have been helping me.