Thin shelled eggs and not laying in the nest anymore...

My 5 chickens are a year and a half old, always have a supply of Layena pellets available and get limited scraps. There are also oyster shells available to them. My GLW has been laying really thin shelled eggs the last few weeks. So thin that they frequently break when they are laid. She used to lay really nice thick shelled eggs, but the problem started when I switched from the oyster shells that look like little rocks to crushed oyster shells. I switched due to cost. She's also been laying her eggs in random places - the floor of the coop, out in the run. She's never done this before. I'm worried she will become egg bound and I'm tired of losing her big beautiful eggs to breakage. Any suggestions?