I'm glad she's doing a little better.
Sometimes as they get older they can start to have problems, but hopefully this is just a slight bump in the road and she will continue to improve.
No, OS doesn't really lose potency, it's good to have it available free choice for your hens.
Extra Calcium like Calcium Citrate with D3 is given for a short period of time (usually 3-5days) and when a hen may be in crisis or having a hard time expelling an egg.
She may be having problems with an egg.
I'd consider giving her extra calcium if she's still acting off in the morning.
You can find Calcium Citrate with D3 in the vitamin aisle of stores like Walmart, CVS, etc. Give 1 tablet daily, just pull down on her wattles, pop the tablet into the beak...