Second trimester loss/birth questions.

Hi, 9 days ago, I delivered my little boy at 17 weeks. We did NIPT so we know there was nothing wrong with him. I had pelvic pressure/pain for 2 days (internal USG showed nothing abnormal) but I started having contractions on the 3rd night. We rushed to the emergency, the scan showed a happy, wiggly baby and my cervix was closed. I was given IV paracetamol for the pain and kept for observation. After 2 hrs, my contractions were worse and every 5-6 mins, and my water broke. My baby was born in half an hour, alive, lived for an hour before passing away. The results so far are all normal, no infection but we are still waiting for the placenta results. I did have a small SCH at 5-8 weeks but that resolved with progesterone (till 12 weeks). Before conception, I had hysteroscopy in May which showed endometritis, and was given antibiotics. We waited till after one cycle before trying, and conceived in June. In your experience, what could be the reason this happened? I am shattered!