Sepsis at birth

What causes this? My son is almost 22 months old now and it still bothers me. Really, really bothers me. My son was either born with or developed sepsis after birth. We stayed in the hospital for 7 days, he was given antibiotics and was treated for jaundice. The first 2 days he screamed all the time. Everyone(nurses) said it’s normal. They said he was a “lazy” eater and would fall asleep everytime he would breastfeed. So we supplemented a bit and I pumped. They did ultrasounds on his kidneys and brain. They found crystals in his kidneys because of the weight loss. He stated gaining and after 3 good days, we got to go home. It bothers me so much still to this day. How did it happen? The doctor just said it can happen and it is very serious, it had to be treated immediately. My son was born at about 2 am Saturday morning and the main baby doctor wasn’t in until Monday. That’s when it was discovered. What if she would have been on vacation for a week? Was I supposed to be given an antibiotic during labor? Anyone have a similar experience or any insight?