Hello,
Using pillows depends on the position that you adopt majority of time. For example if you are a side sleeper, then use pillow to keep your head in the mid position, however you are mostly a back sleeper then use a thinner pillow as if you use a thicker pillow your head would be flexed through the night....
Next Steps
If your neck and upper thoracic spine(area where the neck connects with upper back)is stiff then you are predisposed to getting pain due to stiffness. You need to start exercises to improve strength, maintain ranges and stretch muscles.
Health Tips
have an online consultation to give you some exercises for preventing the stiffness