Hives generally do just what you describe--come out of nowhere, go up and down with no pattern at all, then go away on their own, as mysteriously as they came. Despite their "allergic" reputation, hives almost never represent true allergy to one thing, like a chemical, food, or parasite. I wouldn't worry if I were you. Just take antihistamines as needed and watch the hives disappear over a few days or weeks. If they don't go away in 2 weeks, I suggest you see a doctor.
Best.
Dr. Rockoff