I guess the obvious answer is it must be something I am eating - possibly at 6 or 7pm but I can't think of anything that is all that different from any other time of the day. Neither anything new. Except. Well, apple juice.
I get hives from sulphur and therefore salicylates. Tonight I have eaten ... chocolate, multigrained toast, cheese, tuna, capers (it was nice!) and apple juice.