There could be many causes and more details with a clinical exam is recommended at your dentist or oral surgeon. I am glad to give you some suggestions to assist you before your appointment is scheduled. Your description indicates something fluid filled. This most likely could be the salivary gland duct. Possibly a blockage that builds up and releases when you squeeze. This would appear to be in the muscle area. Other fluid filled cysts may be possible. Trauma to the inside cheek such as biting your cheek could cause a lump. An abscess from a tooth would generally not be in this area. Tumors are possible. Your symptoms do indicate a reaction form stress to the TMJ. There is a soft disc that slides between bone in the function of this joint. It can be out of line or damaged. Muscles can spasm and tense due to stress. Are you aware of any grinding habit? This would need further evaluation at your dental visit. Please try to rest your jaw with a soft diet, good oral care including rinsing and some pain medication such as an anti-inflammatory may help reduce some symptoms. I hope you find some relief and thank you for your inquiry.