Constant pain on back side neck. Neck.

Postural Correction- Sit Tall, Walk Tall. Extension Exercises x 15 times x twice daily - lying on tummy, take left arm up for 3 seconds, then bring it down, right arm up for 3 seconds, bring down. Bring right leg up, hold for 3 seconds, bring it down. Then right leg up and hold for 3 seconds and bring it down. Repeat twice a day- 10 times. Bhujang Asana – Lie flat on your stomach, keeping the palms out, bend the neck backward, take a deep breath and while holding it for 6 seconds, raise the chest up. Release breath and relax your body. Repeat the exercise 15 times twice daily. Core Strengthening Exercises- Straight Leg Raised With Toes Turned Outward, repeat 10 times, twice a day. Back Stretching- lie flat, pull one of the knee forward to chest, hold for 3 seconds, then bring the other knee forward to the chest and again hold for 3 seconds. Then pull both knees towards the chest and hold for 3 seconds. Repeat 10 times each exercise twice a day. Do the cat/cow stretch. Get on all fours, with your arms straight and your hands directly under your shoulders; your knees hip-width apart.

Visit orthopaedic surgeon

It might just be a muscle spasm around the neck also called as wry neck due to improper lying posture or sleeping with head turned to one side

visit a physio for treatment purpose

apply ice and rest the area.Do not go to massage it