I thought you Dr. wanted you to take Cymbalta for pain relief. I am not sure but the other antidepressants don't have the effect on pain that Cymbalta does. If you had that reaction from 1/3 of a pill I'm wondering if you had an allergic reaction to it.
I don't believe it comes in 20mg. The lowest dose in 30mg. If he wanted you to give it a try he should have started you there.
I hope you get the results of your test soon.