How long after an infant is diagnosed with HIE at birth, can subdural bleeding occur?

my son was born unresponsive and was diagnosed with HIE at birth. he was released from the nicu at 16 days old, with seizure medication and close monitoring. At 10 weeks old, he had a CT scan, and a subdural hematoma was found on his right side. He was placed with a VP shunt, and has since been developing normally, with no seizures. They didnt tell us there was bleeding in his brain when we left the NICU. They did an untrasound of his brain 9 days before he was released, and they read it as normal.