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
I had a tetnus shot and 2 weeks later broke out with hives. It was hard to make the connection. Is it possible you had a tetnus shot or some other shot with in a couple weeks or some other medication? Try taking a bath in Alveeno and also Alveeno lotion.