Small of back isn't a 'classic' location for eczema, especially if it's there and no-where else. Might be a dermatitis reaction to sweat, I guess.
If it is eczema, best thing is to hammer it with the prescribed steroidal stuff for a short period, then try to keep it from flaring up using emollient cream. Try reducing your use of bubble bath/soap products for a bit and see if that makes a difference.
Don't use acqueous cream as an emollient. It's just about the worst thing you can use on eczemous skin. I use an emollient cream bought over the counter at the pharmacist and it's made a big difference.
There might still be something in diet. A recurring very itchy rash on the back could be dermetitis herpetiformis, which has been linked to gluten. If you google some images then you'll see how it might be confused with eczema.
Trouble is, there are so many different presentations of different rashes, varying from patient to patient.