Two of the more common side effects of Cymbalta are insomnia and fatigue. Many of the common side effects of medications go away when our body has adjusted to the medication. How long have you been taking it?
It can take anywhere from 1-4 weeks for the Cymbalta to work, so if you've been on it less than that amount of time, I would stick it out and see if the exhaustion goes away. If it doesn't, then obviously you need to see your doctor to discuss this.
Peace
Greenlydia