If left lateral flexion causes pain on the left, then the cause is a muscle, disc, or facet on left side.  If bending head to left and pain is opposite of side of lateral flexion, then cause is opposite side of contraction (right side).  Looking at referral patterns above will help determine which muscle is cause.

To determine which muscle group is causing pain in ROM, determine does pain occur on the side of lateral flexion or side opposite lateral flexion.  If pain occurs during range of motion on the side of lateral flexion then the cause would be the contracting agonist side of the above muscles.  If pain during range of motion occurs opposite side of lateral flexion then the cause would be the antagonist side of above muscles.  During cervical lateral flexion, the side opposite become antagonist and the side towards is the agonist group.