I'm going to quote a relevant paragraph from Gail Demerow's book The Chicken Health Handbook: "Soft-shelled, thin-shelled, or misshapen eggs, ruptured yolks within eggs, reduced production, and prolapse may be the result of either poor nutrition or infection. Watery whites and weak or misshapen shells with altered texture and strength can be caused by a viral respiratory disease, such as infectious bronchitis or Newcastle, or sometimes by vaccination. A coccidiostat in a hen's rations may alter egg size, color, and shell texture."
Demerow also cites Vitamin A deficiency as a cause of blood spots and pale yolks in eggs in chart 2-3.
It's a pretty comprehensive book and this information is all I could find that is relevant to your situation. Jed.