Your Mom should definitely get more tests, despite the CEA results. Lymph nodes can swell out of various reasons, it could be glandular fever for example, so she might want to get tested on the Epstein-Barr virus. Also she should get testes for borreliosis and toxoplasmosis. She should also see a ear/nose/throat doctor, to check for bacteria in her throat. Infections in this area can cause the lymph nodes to swell, around the throat, the neck, below the ears.
Try not to panic about it. I've had swollen lymph nodes in my throat for about three months now and I am still going through all kinds of tests.
My Mom has had cancer for 20 years and is getting chemo. and I have had swollen glands in my throat/neck for the past months and I went through all these tests mentioned above. Next up is a biopsy :)