Hi , given the information looks like there is a reinfection in the tooth that has been treated already. Kindly visit a dentist so that you can get re evaluated, an x-ray will help to identify the cause for the reinfection so that it can be treated and your child can be relieved off the pain. Thinking its a milk tooth that it will fall off, or that an already treated tooth cannot get reinfected is a misconception.
Next Steps
Please visit a dentist for further evaluation.
Health Tips
Take necessary precautions before the pain worsens.