Sometimes there's an acute exacerbation of infection/inflammation in the root canal if the infected tissue inside the canals of a tooth is not completely removed on the first visit and/or sometimes it may be because of a reaction caused by medicinal dressing (soaked cotton pellet) that is put inside your tooth. In either of the cases, tooth has to be cleaned and washed again from inside.
Next Steps
You may have to start Antibiotics and Anti-inflammatory medication. Please consult a dentist. You may call up your existing Dentist for a prescription.
Health Tips
Do not apply heat (hot fomentation). It'll worsen the swelling.