Sounds like the bump is a swollen lymph node. They are there for filtration of infection and become swollen and tender if infection is present. They are also under the jaw line and behind your ears. Your step Mom sounds right. As long as you are on antibiotics it should be OK. They do take awhile to reduce in size after the infection is no longer present, though. If it concerns you, tell your dentist and have it checked by your regular MD.
Hope this helps.
Best to you.
Gellia