You said you were facing a certain problem "now", 2 months after the child was born.
You could have made your decision about how to handle it prior to the child being born. If you'd done that, you would not be facing the problem now, it would be decided. But you didn't.
He asked about why you didn't decide in advance. You got offended by the implication that you hadn't decided in advance. That was something you said yourself, so it's unreasonable to be offended by it, or to blame him for noticing you said it.