I am sure the word "cancer" is in the back of your mind. It is extremely unlikely that this is the case. Drnee outlined the probable causes. You have to get this lump looked at and evaluated by a physician as soon as possible, because encapsulated infections and abcesses progress, and the progression is not a good one. You don't want to turn a minor problem into something that is potentially life-threatening.
Hi
Welcome to the MedHelp forum!
A painful lump on cheek just under the aer could be a swollen lymph node, an infected cyst, a boil, an abscess or an inflamed temporomandibular joint or TMJ.
Please consult a PCP and have him examine the lump. It is difficult to comment without examining.
Hope this helps. Take care!