Do herniated disks always cause pain?

No. Disc herniations do not have to be symptomatic. They may sometimes not cause any symptoms.
No. A herniated disk is a term often used to describe MRI images and can be present without symptoms. A herniated disk on MRI must be correlated to a patient's symptoms. Symptomatic herniated disks most often present as shooting nerve pain down the arm or leg on the side of the herniation.