Dry food will last a very long time from the point of purchase. Like a year. If you keep it an air tight container it will last even longer.
If you still have the bag, it should have a EXP date on it.
It is most likely just a fluke. Cats just puke sometimes. Very small things can upset their belly's.
Try making her a bland diet of boiled chicken and rice. This is very easily digested for cats. Keep this diet for a few days. It is probably just a passing bug that will go away on it's own. You can also give her a 1/2 a pepcid AC to help calm her stomach too.
If it doesn't go away in a couple days, then you should make a vet appointment to rule out any infections, GI problems, parasites.