7 week old kitten really sick help :(

Hi my momma cat had kittens march 20th and all of them.but one seems to be in good health. She is not gaining any weight and feels so small compared to the rest. She hasn't hardly eaten anything in the past 2 days but she is still drinking water. She has been throwing up 2 times once it was clear and this morning it was foamy and white. We are going to try to see the vet tomorrow but can't today because its Sunday and we are pretty scared because we love her

You can't wait!  You need to start syringe feeding her. Get some kitten canned food and some KMR and mix till it's thin.  Then feed her every 4 hours with a syringe, which you can get from the pharmacy.  It's important to get nutrition into this baby NOW.  If needed, call the emergency vet or call your regular vet and see who's on call.  This is an emergency.