Hi Laura, I've gone through these feelings a lot because my Alice died in utero only a few hours after everything looking great on ultrasound (normal blood flow across the umbilical cord, normal fluid, baby moving normally, normal NSTs). The short answer is that no-one really knows; one specialist suggested it might be my BP (which was around 180-190 / 100-110 when I was diagnosed and admitted) because that went really high before it was controlled - but another expert dismissed that as very unlikely. Another I've spoken to suggested that "some placentas are very good at compensating for their inadequacies" - so everything looks relatively ok until the very last second almost and then everything goes pear-shaped big time. Shortly after she died I became very ill too - BP 240/120 (despite multi anti-hypertensives, falling o2 sats, HELLP etc. I know I had a very rapidly evolving disease process; and it all went downhill very fast - Alice dying was just the first sign of that (probably saved my life).
I don't know if this helps you at all but I find it so hard that she died in utero, even after I was in hospital being closely monitored. My daughter didn't die from prematurity like most PE deaths; she died due to the disease. When I'm feeling low I have no confidence for this baby (now 12+4) because I know that all the monitoring in the world can't always change the outcome, and in fact can't even result in the baby being born alive and getting a chance in the NICU :-(
From what I've read, it seems most likely your Cara (and my Alice) died due to lack of oxygen & blood flowing across the umbilical cord, but no-one will ever be really able to give us a definitive answer to this; not at the moment anyway. In my case, I know the situation in utero changed very fast.
Alice too looked very normal when she was born, apart from a bit of peeling around her finger nails and a nose bleed. I had a full post-mortem (because the hospital were so surprised she died like that); that just showed she was totally fine up to the time of death (the placenta was cr*p though).
I hope you can find some way of coming to terms with the uncertainty and grief xx