Egg laying issue

[Beanz chicks]

Hi , I have 6 chickens, all about a year old, all laying eggs. About 3 mths ago my Ameracana stopped laying eggs and only passes an egg sack, no shell. I give them oyster shells, they free range part of evryday. She actualy look like she is uncomfortable. her tail feathers drop for a bit, looks like she has cramps. Then tail feathers go up and she seems fine.
Any ideas?

 

[SueT]

Welcome to BYC....Well I don't know but maybe someone will answer who has an idea. They might want to know, Do they get layer feed? Are all the others laying regularly?
Also, if you get no advice, you might want to post under diseases and emergencies forum...Good luck!