Same here - ds had to have two teeth out last year when he was 5, I have never had any fillings and we brushed his teeth religiously, he drank only water, no sugary snacks, etc. He had them out under GA and they discovered a hole in the one next to one of them aswell, so he had 3 teeth out!
I was devestated like you, felt like the worst Mum for a while. I bf ds till he was 3, and my dentist said he'd seen a lot of anecdotal evidence that bfing was the cause. He said 'Humour me, did he have a favourite side?' (He did.) 'And I bet it was the right, so the milk pooled in his mouth on his left where the decay is?' No, actually, so that blew that ridiculous theory. 
After he'd had them out and we were waiting for them to call us in, the staff at the dental hospital sat us down and said 'Now you will make sure you brush his teeth twice a day from now on, won't you?' I wanted to cry with frustration - no one could explain why with careful brushing he had such decay, they just didn't believe us! I think they were convinced we had given him coke in his bottle or something. My niece the same age went to bed with juice in her bottle every night, and has the healthiest teeth!
We use adult toothpaste with him now, and - touch wood - we are a year on with no more decay .