Thanks for asking the query,
and vomitting can be due to use of antibiotics, you can stop the use of antibiotic for sometime.
As you mentioned Re-infection of the root canal
treated tooth has occured, i would suggest you to get clinical examination of the tooth, take an x-ray, depending upon which treatment will be decided.
Meanwhile you maintain a good oral hygiene
, use lukewarm saline gargles 3-4 times a day.
Use antiseptic mouthwash gargles twice daily to prevent infection and promote healing.
Hope this helps out.