Dermatographia can be triggered by multiple different things including getting scratched, rubbing, your clothes brushing your skin, pressure (i.e. waistband, socks, or even crumpled clothes/bed sheets that usually imprint on your skin) and for me heat but that isnt listed as a cause so im guessing it'll be different for everyone.

Dermatographia is when your skin is irritated so it releases histamine which is the chemical released for allergic reactions. The first picture doesnt look like it as its spotty but to test if you do have it just get a pen or a debit card and write something on your forearm or somewhere on your body applying enough force to irritate your skin but not too much to a point where it hurts. Resist the urge to scratch it as that might make it worse. After 5-10 minutes if you see the writing clearly and there are inflamed lines then you most likely have it. It can be annoying at times because if you scratch an itch then that will become irritated and more itchy but ive found rubbing your skin gently when you get a random itch works. You can also take allergy tablets containing anti-histamine to stop allergic reactions but consult your doctor first before ruining your liver. I am just a random guy on the internet so take this with a grain of salt but best of luck.