Without a description of lesions it is hard to know what this might be. The 2 most likely conditions would be dry skin or lice, despite your good hygiene. Lice could be picked-up in a hotel/motel and not be detected because they are fairly invisible.
Hi. If, when you say "at night" you mean when you're in bed, it sounds like you could be allergic to your bedding (i.e. to one of the fabrics in it or to the dye or some other chemical that is added to the fabric). Have you recently replaced your sheets or blanket? I'd say it could be a laundry detergent or fabric softener, but unless you've only used that product on your bedding, I'd think it would also affect you whenever you're wearing clothes. One clue that it could be your bedding is if you wear pajamas our something and your forearms are the only parts of your body that are actually exposed to your sheets.
Do you have any other symptoms accompanying the itching forearms?