mifta
Just a long-shot but could I ask your age (approx) ?
It's just that since the menopause I have had a consistently itchy back with nothing visible. I have put it down to 'formication' (please note spelling!!) which is the sensation of ants crawling over/under the skin.
I discovered this is a known sensation after menopause with no real explanation. It has got a bit better so I hope it will disappear completely one day!.
Hope yours goes, it's irritating...pardon the pun!