One single itchy bump on forearm. disappears after 30mins, reappears next night. 1in diameter. no ring, no heat, pink in color. ?

Difficult to say: Could you please post a photo of the lesion so we can have a better idea of what it could be?

Photo please: It is difficult to comment without seeing a picture of the lesion.It could be an insect bite, a cyst, a fungal lesion. It does sound quite unusual that it disappears after 30 minutes. Urticaria is another possibility as well.

Contact dermatitis: Most likely, this is called contact dermatitis--usually due to an allergic reaction to some chemical your skin is in contact with during the day. The most common is a Nickle allergy. Nickle is used to harden many metals, including the steel on the back of your watch.