concerned parent

[nmgalle]

My 6 yr old went to the dentist 2 weeks ago they did x-rays and a cleaning and said she had no cavities. Then a week later her face was swollen we went back to the dentist and they took another x-ray and tell me she has an abscess tooth and she needs a root canal. We just completed the antibiotics and she goes back tomorrow. The affected tooth is one she had a filling in 6 months ago and is her baby molar. My questions are 1. Did her dentist mess up and cause this when he filled it? 2. How could he not see decay in the tooth literally 1 week earlier? 3. And if he messed up how can I tell if she actually needs the root canal?