We recently switched my dog's food and now all of a sudden she has become weak and not moving much. What can I do?

Backstory: My dog is 14-15 years old. She's been healthy up until February. We had her cancerous ovary removed. That didn't make her happy but we treated her with the medicine and she's been good.

We recently switched (not on purpose) from Purina Beneficial Healthy Weight to Beneficial Healthy Fiesta. That was on Thursday. Yesterday she did not come out of her pen. I chalked that up to the heavy rain and occasional crack of thunder (common for her to do that). Today I go to let her out and she walks out slowly, going to the front yard (100 yards or so) and hasn't left the spot I also saw that she had thrown the food up (what little she had eaten overnight).

I've been feeding her some wet Ol Roy canned food and an occasional pretzel (her favorite). Her stomach is still gurgling and she hasn't moved. I'm sitting here and hand-feeding her with a spoon as best I can and a bowl of water is nearby.

Aside from the obvious "take her to the vet" which I am not sure is possible around here, how can I nurse her back to health?