I have been taking antihistamines for my hives (entire body), and they are working. but now i developed a small area of skin that's itchy and dry, and it has gotten bigger every day because i scratch it in sleep. can this be caused by antihistamines?

No: As you have suspected, it is likely from the scratching. Consider taking your antihistamine up to 4 x daily to control the itching. You can also switch to a different antihistamine if you suspect that your current antihistamine is causing the problem. In the meanwhile, leave a bottle of anti-itch agent at your bedside to control the itching during the night.