I get the same type of dermatographia reaction from just touching my skin a lot. Also, I can break out in hives from seemingly nothing. Doesn't happen very often, though.
Overall, though, I am a very allergic type person, despite the fact only a couple of specific things show up when I get tested. I developed a shellfish allergy in my mid thirties, after growing up eating tons of lobster and clams, shrimp. I can eat scallops, though. I also couldn't drink wine for about ten years (sulfites), but all of a sudden, that went away (I tested it one year at Passover, and when I didn't die from drinking the 4 cups of wine, I decided I was OK). The shellfish allergy is related to my allergy to IV contrast dye/iodine, which I found out about when I had a CT scan. I suspected I might had this, as my mom did, but they convinced me to have the dye. Within one minute my eyes swelled shut and I had a huge welt on my wrist.
I also am allergic to Thimerisol, the preservative that everyone thought caused autism... it used to be in contact lens solution and after like twelve years, my eyes started swelling up. Now, I can't get certain immunizations, because they have Thimerisol as a preservative.
Go to an allergist.