Most probably a TMJ (Temperal-Mandibular-Joint)issue.
It may come from an unbalanced bite, which might be able to be fixed by your dentist by just a little grinding down of the surface of the teeth that are hitting (the opposing teeth) first, i.e. before they hit on the other side of your mouth (making both sides occlude at the same time).
Or…
You may be grinding/clenching your teeth and need to wear a nightguard when you sleep.