If I were you I would ask the people at the place where you are doing community service, for suggestions on how to help your mom cat. They may know of somewhere you can get veterinary help for a very reduced rate or for nothing.
With your mom cat, it could just be that she is not ok with the food. Can you tell if there are any other behavioral signs that would indicate she is sick? How is she doing with nursing the kittens? How is she doing with drinking water and fluids?
Someone earlier said it is important for her to see a vet. Indeed, if she has diarrhea she will dehydrate quickly, especially nursing the kittens and she will go down quickly.
Possibly the shelter where you are doing service can help you give fluids to the mom cat. The best thing would be to see if they can help you with a vet.