Thanks for asking the query,
Cheek swelling is not related with the root canal
treated tooth as it is on the opposite side.
The swelling can be due to allergic reactions, can be a symptom of underlying dental isssue on the opposite side of the face, can be due to inflammation of lymph
nodes or parotid gland
known as Parotitis
I would suggest you to go for thorough clinical examination, if nrcessary go for OPG radiograph which may be helpful in confirming the underlying cause.
Maintain a good oral hygiene
, use lukewarm saline and antiseptic mouthwash rinses.
Start taking with multivitamin
Hope this helps out.