Hey hi, the white wound you're describing near your back teeth is likely a major aphthous ulcer or an infection related to an impacted wisdom tooth (Pericoronitis). Since it is spreading and causing pain, you need a physical exam to check for infection
Also if available attach xrays of ur teeth.
Next Steps
Consult a dentist immediately for an IOPA X-ray of the area. You may need a course of antibiotics or a professional cleaning under the gum flap
Health Tips
Rinse with lukewarm saltwater 3-4 times a day.Avoid spicy or hot foods.