This is a thyroid board, so maybe not the right place. I'll take a stab at it though.....
Maybe a salavary gland problem, like a stone in the salavary gland. I would google that and see if it fits. I would also contact a GP about it as I have heard it can get painful? Good luck and hope you feel better soon.
You may also look into lymph node swelling
That is = as well - very painful.
I just thought about something else.... infections of the mouth (like abscess of a tooth) can cause that kind of reaction too.
I would say swollen lymph node, as well, though this doesn't always have to be painful. Is the lump soft and moveable, or hard? The swelling under the right ear may very likely be a lymph node, too...but it doesn't make sense that it would disappear 15 minutes after you have eaten. Have you had either of these things evaluated by a doctor, potentially a specialist like an ENT?