I looked up the side effects of Cymbalta, and itching IS a side effect. Some side effects can occur at any time you take a medication.

I'm sure you are very uncomfortable
I don't remember if Benadryl works for you or not...

If the Benadryl doesn't work, and the itching is really bad, I would try skipping a dose and calling your pdoc as Mari suggested

I know that you don't feel like the med is helping you anyway...