Zoloft is an SSRI (Selective Serotonin Reuptake Inhibitor), meaning that it makes the serotonin you have more available in the brain. More serotonin = increased mood and less depression.
Cymbalta is an SNRI (Serotonin and Norepinephrine Reuptake Inhibitor). Thus, it affects both serotonin and norepinephrine. Norepinephrine (or noradrenaline, as it is sometimes called) is a neurotransmitter like serotonin (delivers messages in the brain), but it is also a hormone that produces energy in times of stress (think "fight or flight"). So, people with depression that makes them tired or apathetic often respond well to an SNRI because it gives them that shot of energy they need to get going. People with pre-existing anxiety? - Not so much
The only reason I can think of that the Cymbalta was given is because of the muscle tension you were describing. It's often used for pain management and stress in fibromyalgia and the like. However, for anxiety with depression, a straight up SSRI might be better (Prozac, Celexa, Paxil, Zoloft, Lexapro)... Also, the heart pounding might be anxiety, but it could possibly be due to the Cymbalta too, so you might want to give you NP a call.
Does this answer your question?