Offer the oyster shell free choice - put it in a small container in the run or scatter it on the ground. They will take what they need.
Stress can be seen with egg quality issues, going off feed, staying by themselves,etc. It's hard to know when they are stressed because chickens can hide an disturbances well.
Now, any of the above can be a sign of illness as well, so observing the flocks behavior - knowing your hens will help.
Since she is young (10months) she may still be working out the kinks. You mention that she is not consistent with her egg laying - one day thin shell, another normal, another with calcium deposits. Hopefully providing the oyster shell free choice, seeing that she is not being kept away from food/water and no one is picking on her, the problem will resolve itself. Also limiting treats (scratch, corn, etc.) to no more than 5-10% of daily intake may help as well. Offer a small amount of leafy greens, veggies or fruit daily as a treat.
Keep us posted.