Are you a one-shoulder bag wearer?
Another way of getting a stiff neck is by wearing your heavy bag on one shoulder. You know what, be it on your bag or at your side with a shoulder bag in tow, carrying something that is very heavy can cause stiff neck. This is a fact.
As for wearing your bag in one shoulder, the problem with that is with your posture. The trapezius muscles are strained in one side because of the heavy bag. Again, you don’t need that strain. Just lighten the load since doing this can make you feel better, overall. If not corrected, you could have problems with your spinal alignment, you know.