I don't know if you can use Calcium Lactate or not.  Let's ask 
@azygous
I do know  you can use Calcium Citrate.
Is her crop empty first thing in the morning before she eats/drinks?  If it's staying fully and squishy, I would treat for Sour Crop according to this article 
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
Getting Calcium into her is a good idea as well, she may be having problems with another soft shelled egg.  It's possible that she has laid internally as previously suggested in the other posts.