Dogs can vomit for so many reasons, often just down to a change of food as you mention or eating something that doesn't agree with them. Sometimes it can be a sign of more serious disease, that needs urgent veterinary care.
If the dog is otherwise bright, withholding food for no more than 24 hours in an adult dog, then introducing a light diet is often enough to settle things. However if the dog is lethargic, unwell, has loose stools and refusing fluids or vomiting them straight up, you MUST see a vet.
No one can diagnose your dogs problem, ring your vet for advice. If they want you to bring her in, you have a duty of care for her and you must somehow find the money, even if that means borrowing or using a charity vet care centre if you can.
When a dog is ill it deserves the best care possible and as owners we have to be able to arrange that. Please consider insurance, though there is an excess to pay, so you always need to be able to pay this.