Well, this is a long shot but it is a possibility: Celiac's Disease can cause itching in odd places, and at odd intervals. My stomach itches like crazy when I ingest gluten, and there are signs of dermatitis herpetiformis there, though it is most commonly found on other areas of the body.
There are other symptoms, as well, ranging from weight loss to malnutrition to pain/gas/bloating/nausea, but some people show no physiological signs of suffering from Celiac's at all - none.
The problem is that many doctor's aren't familiar enough with Celiac's to suggest a blood test, the blood test panel is expensive and wasn't covered by my insurance (which is really good insurance otherwise), and even though blood tests are vastly improved nowadays a positive result is not enough to confirm the disease - a biopsy is generally required to assess damage to the small intestines.
I would certainly read up on it and determine if there are other symptoms you've been having that fit the description and just aren't aware that they may be related. Excess gas or weight loss can certainly be a result of diet, after all! If you think it might be the case that this is the cause then find a gastroenterologist who is willing to run the blood panel for you. Better safe than sorry at that point as continued damage to your body is not a good thing and will eventually cause permanent problems.
I hope and pray it's something much simpler and less life-changing. I really wouldn't wish this one on any one.
Perhaps a recent change in laundry detergent? Lotion? Even a change in hand soap - especially if someone else touches your back - could cause a reaction you weren't aware of.
L.