She probably needs some electrolytes and the original Gatoraid or if you can get the one with Cane sugar is a good fluid booster. (My daughter that has something similar to POTS has to have salt and a good bit of it.......my daughter had a stress test and she was their fastest fainter ever. She didn't have to do the test. They determined that when her BP has to go up, her heart gets out of sync and that is why she faints. If she keeps her fluids up, takes her BP med for low blood pressure and salts her foods, she doesn't have episodes.)
I hope your daughter feels better. That is a really weary feeling to not be able to move with that drop going on.
40 as the bottom number is very low and she needs to be sitting or laying down to prevent any falls should she faint.