Not sure if this is what you are seeing, but I've had 2 blue cochin hens go lame on me in the past month. Not sure if yours is having a similar issue, but this breed seems to me to be at an increased risk of aquiring this sudden lameness from a lack of calcium, or perhaps it is the vitamin D that allows absorbtion of the calcium. They can have muscle cramps or, in the case of my hens they were each laying flat out when I found them. I immediately isolated the hen when I noticed the problem, and gave vitamin enriched water, increased the oyster shell in the feed and fed the best calcium foods I could find...ie spinach etc. Both birds recovered in a couple of days. They had each layed a soft-shelled egg just at the onset of the other symptoms. One was probably stress related, as I had just brought her home after buying her. The other had access to oyster shell, so I'm not really sure how it happened that she was deficient.
Anyway, just thought I'd mention this as a possibility...