Also agree with Speed Racer. If you can't get hay for another few days, you need to give her ample forage replacement of some kind, because 2 flakes of alfalfa(however big the flakes) is just not enough. I agree that it's not a good idea to feed too much of this, as it's a rich feed, but it would be better than nothing to give her a lot more if you have no alternative. It is very bad for a horse's system to allow them to go hungry for too long.
Regarding too much/not enough grain, SR was talking about too much in one go(I'm assuming, SR? I also don't know what a gallon looks like - we're metric over here!).
Horses have very small stomachs(about the size of ours) and very quick metabolisms. They are designed to eat small amounts near constantly and don't deal well with big &/or infrequent meals. On top of that, grain & other starchy/sugary feeds are not generally very good for horses anyway, and due to the quick metabolism, unless well processed first, tend to pass through the stomach into the hind gut without being digested. Even if processed, if large meals are given, it will pass through the stomach undigested, because the stomach automatically empties when it becomes more than 2/3 full(about, from memory). The big quantities passing through are what can cause impaction colic, but also the ensuing hind gut acidosis this causes can lead to ulcers, laminitis, colic and weightloss.
So... if you are going to feed grain, ensure you feed the ration over as many small meals per day as possible. Oats are about the safest grain. But I don't personally recommend feeding it, or other high starch/sugar feeds, for the above reasons and also because hi-carb rations also frequently cause insulin resistance(like type 2 diabetes), which is also a leading cause of laminitis and can cause weightloss, etc too.