Hi Cassie,
I don't have any wisdom to impart, but I didn't want you to feel like you were alone
I'm sure it was nothing that you did, but the post mortem will probably help allay your fears more than anything I could say.
And if it were something you'd done (which I SO doubt) you would know for the next pg, right? If there were some reason you couldn't carry to term then there would be precautions you could take that could help, like a stitch in the cervix or whatever.
I hope you get closure, RIP little Ryder