I've never had this happen, but my mom had something similar. The ENT said it was because she would get dehydrated and the fluid in her salivary glands got sluggish and thick. It would clog the openings, then the fluid couldn't escape and the glands would swell. Usually, she'd end up getting an infection from it that required an antibiotic. He would use a teeny tiny hook to clear the openings. Then at home, she was to apply heat to the outside, massage the glands, and drink plenty of fluids. It would take days for the swelling and pain to disappear.
Not sure if any of that helps. But I hope you figure it out and get feeling better!