Your condition is called dermographism, which is a type of urticaria, or "fong mok" in Cantonese. Sometimes, you may get big, swollen patches of skin-coloured "fong mok" on other parts of ur body. It is usually aggravated by heat, stress, exercise, minor pressure/friction/scratching.

So, don't scratch it as u'll get more of it. Leave it and it'll subside. If it becomes recurrent and bothersome, u can try antihistamines like Cetirizine, Loratadine, etc, easily available OTC. It is a benign skin condition, so dun worry, but may persist for some time. Good luck

Lupus rash is different in which it does not subside within a few hours of onset. Lupus rash only resolves with steroid cream and leaves a pigmentation. Lupus is also associated with other symptoms like prolonged fever, fatigue, oral ulcers, hair loss, joint pains and so much more. So its quite unlikely.