anyone know what causes this?
It is hard to tell what might be causing this just by looking at the picture.
You may be having an allergic reaction to a food, a medicine, a cosmetic, the fabric of a particular piece of clothing, the washing powder you are using or even the water in your house (due to some excess minerals).
This could also be a nervous hives.
Anyway, there are many guesses, so you would need to see a dermatologist (especially if these hives appear chronically) to investigate the cause.
It can be alot of factors, the state of mind, the weather, eating habits, etc. When i had this issue it was certain fabrics.