Hello Sue and Welcome to Back Yard Chickens. Glad that you have joined this fun and poultry loving community. Sue I don't know what could be happening with your Pea chicks but I've seen this same thing in one breed of chickens that I raised. It seemed to be genetic with my chicks. Once a lot of them had died off it stopped for no reason for the loss or the one that survived and were healthy.
Hiya, Sue, and welcome to BYC! I wish it wasn't on such a sad note. I see helpful links above and hope you find what's happening so you can try it again.