Re: Trying to determine the cause...
I hope you're having a good day today. It sounds as if yours may truly be muscular in nature. Posture can have a huge effect on the entire body. Have you looked at your hips and shoulders? Is one side higher than the other? Many muscles can tighten/shorten and pull on the jaw. The sternocleidomastoid is just one of those muscles that can pull on the jaw and cause pain.
Unless you are noticing a difference in your bite or clicking and popping within the joint on one or both sides then it is probably muscular in nature for you. I would work to address your posture. In addition to your posture it wouldn't hurt to address any stress you may be having. Stress can cause clenching which can also hurt the muscles of the jaw. If clenching is found to be an issue for you, a splint can be used to help try and stop that from happening at night but it would be best to address the cause of the clenching. Certain medications have been known to cause clenching so be sure to make sure nothing like that could be happening in your case.
Are you making sure to give your jaw a break? Make sure to eat soft foods and use moist heat on the joint area and muscles when you have a chance.