I do feed them oyster shell. What else could it be?
If you are feeding are good layer feed then their calcium requirements will be fine. Oyster shell should only be offered on ' free take ' , the hen will know if and when to take it. Once the calcium level exceeds 3% your birds are at real risk of health issues! gout for one.
This link is graphic , but it shows the possible results of excess calcium and or protein.
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/232/gout