I'm not sure about the egg being cracked before coming out, but good thing it came out instead of breaking inside of her! But, I know what you are talking about regarding the porcelain sounding/feeling egg. I have a hen that lays one like that. Are yours fairly easy to crack? Mine are. I don't know what's wrong. She used to lay good hard shell eggs & now she doesn't.
Mine gets free choice oyster shell along with layer pellets that has calcium in it.
Mine hasn't been laying due to either molting or the shorter days or colder weather or a combination of all. I'm hoping when she starts back laying that she will have more normal shells.