Enlarged lymph node on cat

Question
Two weeks ago my 16 month old spayed female cat went to the vet for a comprehensive exam and annual booster shots. She received the following vaccines:
Feline distemper combo, feline leukemia virus, FIP, and Giardia feline. Her routine exam turned up nothing. She didn't eat for about a day and was sleepy, but after about 24 hours she was back to normal. A week after the vaccinations I discovered an enlarged lymph node under her left arm (about the size of an acorn). Took her to vet and he he suspects it's a reaction to the vaccine. He wants to wait another week before we do anything further. She acts and eats and plays totally normal and seems 100% fine. I am worried though. Could it be related to the vaccine? Is this common? What could cause this enlargement and how long could it take to go away? Should I be concerned that it's lymphoma? How long should I wait to have it biopsied? If it is cancer would she be demonstrating other symptoms, and would it appear that large out of no-where, within a weeks time?
Thanks for you help, and sorry for the long details.
(She is negative for all diseases and has no other enlarged nodes)


Answer
Sincere apologies for the delayed response.

Infection is the most common cause of lynphadenopathy (enlarged lymph nodes).  However yes vaccines can cause this especially in the left axillary region since the Feline Distempter vaccine is usually administered in the left shoulder region.  I would say with the timing this is a very likely cause and you should give another week for its return to normal.

Lymphoma would be another possibility and should the lymph node remain enlarged for significant time I would suggest having her seen again to check for that or some undiscovered infection, however given the history a vaccine induced lymphadenopathy does seem more likely.

Please let me know if I can be of any further assistance and again apologies for the delay.