My husband has had numerous post radiation infections that looked like this. They always present on the same side as his cancer. His surgeon said it was swelling of his parotid gland. A few weeks on a strong antibiotic usually wipes them out. Get to a doctor asap as they can quickly get out of control. The first one he had occurred within a week of completing rads and he had to be hospitalized with IV steroids and antibiotics.

Good luck!