Hi! It is difficult to comment without examination. If your mother recently had fever then this could be a parotid swelling. Also it is possible she may develop fever soon. Painful lump under the ear could also be an abscess, folliculitis (hair follicle infection), an infected cyst, an inflamed lymph node or a neurofibroma (localized harmless swelling in of nerve and connective tissue). It could also be TMJ. Please consult a doctor. Take care!
That has happened to me a few times before. In particular instances, It came on very unexpectedly (I was not feeling sick), very rapidly, and the swelling was not only below my ear but also along the side of my face. I believe you can get swelling and pain in this area for several reasons. My eustacian tube (the tube that links the pharnyx to the middle ear) became blocked due to infection. Not only did it cause pain and swelling, but it also caused a bit of vertigo. I was treated with antibiotics. The times that the swelling and pain came on very rapidly, it turned out that I had some very small glands inside the sides of my mouth that were blocked. The doctor ran a very tiny instrument in and out of the glands so whatever was blocking them drained off. The pain and swelling subsided about as quickly as it started. Good luck to Mom!!
Is there seeping? Is the inner lining of the ear dry? Does your Mom swim or use a cue-tips a lot? If so it could be swimmers ear ( infection of the ear caused by excessive moisture or dryness. The cure for that is an ear syringe for the wax blockage and some antibiotics for the rest. Followed up with daily moisturising of the ear afterwards...
For me that was the problem/soluition. Hope this helps, Tc/H.