Soft Shell

Hello
My BLack Star looked uncomfortable last night, just before lights out we found her in the nesting box but she did get up when we opened the door so checked her over and but her on the perch where she stayed after checking on her an hour or so after. This morning she has laid a soft shelled egg.
Why do they lay soft eggs? She has been fine up until now, but I am worming my Hens at the moment and wondered if this has anything to do with it.
Any help will be gratefully received
Thank you

Thank you Joyfull
I do give them mixed grit with oyster shell but I will get some extra oyster shell today.

Off to the local pick your own farm to get some cabbage, oh and while I'm there I might just have to get some stawberries for tea, well I have to make the most of the petrol cost and as my stawberries aren't ready yet!!