Thanks for posting your query.
The neck pain may be due to prolonged working time in dental chair during root canal procedure had undergone.
You can get physiotherapy
done regarding the neck-pain.
Also check whether root canal treatment
had undergone by removing infected pulp completely.
Rinse mouth frequently using warm saline and betadine solution.
Use frequent hot packs.
If sinus infection
persists,please do visit your dentist
and get it drained completely.
Maintain oral hygiene well.
Take complete course of antibiotics too.