Besides tmj, is there any other differential diagnosis for pain in masseter?

Referred pain: You can have referred pain from trigger points inhe shoulders and back.

Tooth grinding: It could be because of night time tooth grinding or it could be something wrong with your parotid salivary glands that are located in the same area.

TMJ Dx: Could be related to nocturnal bruxism this could be related to sleep apnea. If you snore you have about a 50% chance of having sleep apnea, morning headaches are also a common sign of apnea. Obstructive sleep apnea (osa): osa is the most common form of sleep apnea. Obstructive sleep apnea occurs when the airway is blocked or obstructed during sleep. This can be caused by tissues in the through su.

Other causes: Can also be masseteric spasms due to CNS overactivity, salivary stone blocking "stensons duct" which emerges in the mouth but travels over the masseter muscle, benign growths within the muscle and direct traumatic occlusion irritating the muscle.

Yes: Clenching and grinding can cause spasms in the masseter and the lateral pterygoid muscles. Also, an inflammation of the parotid gland can cause pain in the masseter. Beyond that you are looking at any other infection or tumor. Likely the first 2 mentioned, especially if the stress level is high. See your Dentist for evaluation if it persists. Good Luck.

Local Myalgia: Local myalgia includes muscle co-contraction and muscle soreness. See an orofacial pain practitioner for evaluation and treatment if needed.

Yes, many things: Many things, but by far most likely is clench/grind related to TMJ. If so, maseter muscles often enlarged. Many structures in region. If you're concerned about all the possibilities, see an ENT doc for a complete head and neck exam.