Cymbalta affects two neurotransmitter systems. Norepi reuptake and serotonin reuptake.

Serotonin, is a vasodilator, and most of it is in the body. It is a biogenic amine with many actions. So Cymbalta affects the body as well as the mind.

Serotonin is complex and does many other things as well:
http://en.wikipedia.org/wiki/Serotonin

But common side effects with drugs like Cymbalta...reflect the increase of serotonin in the gut (nausea/diarrhea) and skin, vasodilation. People with Carcinoid ( with high serotonin levels) have very severe flushing, as one symptom. This comes with a feeling of heat.

The norepi part, of the Cymbalta, raises blood pressure, and stimulates the sympathetic system to some extent. The effects on the bladder..causing urinary retention ...is part of this action.

Here is the Wiki on it:
http://en.wikipedia.org/wiki/Duloxetine

It is very difficult to anticipate how this complex drug will affect people. Depending on your particular genetic make-up regarding neurotransmitter balance, you will have certain side effects and not others. Some people are "high serotonin" types, and therefore their side effects would be different than the lower types. High serotonin types often discontinue SSRIs and SSNIs (like Cymbalta) due to severe side effects.