Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about your girl Carmen having an abrupt change in foods, and now experiencing diarrhea.
Eating a different food can cause stomach upset itself. It would not be unexpected to see some nausea, vomiting, loss of appetite, loose stools or even diarrhea.
You can try a few things to try and settle her stomach.
If she seems at all nauseous or not wanting to eat to settle her stomach today you can try some acid reducers. You can give either:
1) Pepcid-ac (famotidine) at a dose of one quarter of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one quarter of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours.
These are very safe medications and can be used for several days if necessary.
I would not feed her anything for 12 hours to rest her stomach. Offer ice cubes to lick to keep her hydrated. If all goes well with that offer her small amounts of water later and see how she does.
If that goes well offer her a small amount of a bland diet tomorrow morning after her food fast.
A homemade bland diet consists of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled, plain, white rice, pasta or potatoes. You can add probiotics to replace good bacteria often lost with propulsive motions of diarrhea. Good brand name products include Fortiflora, Benebac and Proviable.
If that goes well feed her this for a few days until her stomach is settled, then slowly start mixing in her regular food, gradually increasing the amount of regular and decreasing the amount of bland over a week's time until she is converted back to all dog food.
If her diarrhea continues today even with not eating you can start the kaolin/pectin formulation of Kao pectate at 1ml per pound of body weight every 6 to 8 hours. This coats the irritated intestines, absorbs bacterial toxins and should help slow the diarrhea. It is quite safe and can be used for several days as needed.
If she tries to vomit with no success, or vomits repeatedly and cannot keep anything down, doesn't eat for you after her fast and seems more lethargic then she should be checked by a veterinarian. Very rich foods or ab abrupt change can cause a bout of pancreatitis, or inflammation of the pancreas causing increased enzyme production and release leading to severe abdominal pain.
Please let me know if you have any further questions.