I keep getting hives at night time. Anybody know why?

About 2 1/2 years ago, I started getting hives out of nowhere. They lasted about 11 months. It was so bad that my entire body was covered with welts on a daily basis and about 3 times a week i'd get swollen lips... I had blood work done. I avoided all foods that I was allergic to but the hives still came and went. The last year and a half I have been have free. The last 3 days I have been getting the same kind of hives as before, but they are extremely miniature and only start coming out at nighttime after 9:30.. What might be causing them to come out at night? I'm really confused...