I have been experiencing what feels like fluid retention/swelling underneath my jaw almost everyday for the last ~6 years. The swelling lasts throughout the day and I don't notice any triggers. The degree of swelling can vary throughout the day and week. It can get even worse when I'm not drinking enough water, but even when I do drink enough water it's still pretty bad. I also keep a low salt intake. The swelling can go down a lot after a hot shower, only for it to swell up again later. Doctors found nothing after feeling my jaw/neck. There were a few times when the pressure was just absolutely unbearable, where I wasn't able to lie down, but it went back to the usual degree of swelling the next day after getting my period. But it still swells up even when I'm not close to my period. I do not experience any pain, aches, or fever alongside the swelling. The discomfort is from the pressure that I feel when the area underneath the back of my jaw is swollen. I do not feel any lumps or anything hard, either. From the outside, it just looks like extra fat around my face, but on rare occasions the swelling goes away and my face looks more defined.
My blood test results showed that I appeared to be immune to mumps. I also just did an MRI with contrast and the results came back normal, as well, although the ENT said that I had a third molar.