Hello and welcome to Healthcare Magic. Thanks for your query.
node enlargement is not a known side effect of lithium
and so, is unlikely to be related to the medication. It could be the result of any infection or inflammation in the throat, mouth, etc.
I would advise you to take her to a doctor for a physical examination and evaluation, in oder to determine the cause of the lymph node enlargement. Further treatment will depend on the cause identified.
Wish you all the best.
Dr. Jonas Sundarakumar