Cancer doesn't suddenly blow up in minutes. That sounds utterly miserable, though, and I hope you feel better soon. I would guess a salivary gland stone? Maybe when you try to eat, the saliva flows but has nowhere to go, so the swelling worsens?
I'm not a doctor, just a hypochondriac
So obviously don't take my idea as anything important, but if that's what it is, it's not serious, just really painful. The fact that the doctor let you go right home says that whatever it is, it probably isn't serious.