How long has she been laying??? Her eggs were always good quality before???
Besides oyster shell (which isn't an automatic miracle cure...it takes a little time to absorb nutrients), you can also give her crushed up egg shells from your other hens, yogurt, etc....anything high in calcium. I'm not familiar with layer mash...is it formulated specifically for layers??? High calcium levels?? I might contain her for a few days so that she can only eat things high in calcium, and see if things improve. I think they all get a glitch from time to time; I wouldn't worry yet, but if she's still doing it a week from now (after you've tried a few things), then I'd suspect something else.