I'm not sure which glands you're referring to, but I had this issue with the salivary glands in my cheeks/near my ears. Started 6 months after RAI, lasted about 4 weeks, then mostly subsided. I ended up with a blocked gland on one side--they don't really swell much anymore. (I also had 150 mci.) My ENT was able to feel the "block," or "stone," and checked it by blowing a mini airgun inside my mouth to check for salivation.
My guess is the antibiotic is in case there was/is an infection.
There are some threads on salivary glands here, that might be useful to you. Hope it is temporary for you!!