The following is probably no more than an irrelevant anecdote but I'll post it anyway - as it just might be relevant to you:
Many years ago, my mother mentioned to me that my father had been getting lots of rashes and itchiness on his legs, just at the top of his socks. She put it down to having recently switched to using a biological washing powder and hoped that going back to a non-bio product would resolve what she took to be an allergy problem. However that didn't seem to make a lot of difference and my father's mystery problem continued.
It was several months later, when I was visiting their house, that I realised that the problem had started at a far more significant moment than when simply changing washing powder; it was when they'd moved house. So I took a very close look at the carpet in their living room and found that it was heavily infested with fleas (which had presumably been introduced by the previous owners' cats). All it then took to solve my father's problem was to spray the carpet with flea killer.
As I wrote above, that story probably isn't relevant in your case but if you've got a cat or a dog it might be worth spraying your carpets with a suitable pesticide, just in case!