Wow! That was very negligent of the dentist! He should have told you about it, told you what his thoughts were and let you decide. Cavities in baby teeth can cause issues in the permanent teeth. Kudos to you for finding a new dentist!!!
turns out it really was a cavity and the dentist claims he noticed it but wasn't worried about is since it was just a baby tooth. But now that it is bothering her he decided it was time to take care of it. Looks like we are on a dentist search again!
Before assuming anything call your daughter's dentist and ask him what this could be and go from there. Cavities take time to form so talk to your dentist and see what he has to say. Best wishes.