Hello Susan, I'm Dr. Deb and I'll do my best to help you today.
I'm sorry about this concern for Missie.
It's certainly possible that a change in diet could trigger looser stools. I don't worry too much about the change in color unless it's black, lots of blood or white. And the mucous/slime reflects intestinal irritation and is nothing to worry about either.
There are other possible causes of diarrhea, of course, including:
1. internal parasites such as whipworms, hookworms, giardia for example. Stool samples are not always positive when these parasites are present since eggs are not shed in every sample. So, a positive is a positive but a negative is not necessarily a negative, if that makes sense.
2. dietary indiscretion..she ate something that is irritating her intestines as it passes through.
3. inflammatory bowel disease
4. ingestion of spoiled or rotten food
5. viral or bacterial infections.
6. Systemic disease
...and this is just a partial list.
If she is otherwise acting fine, then you may be able to treat her symptomatically for a few days and she will get past this.
1. Continue to feed a bland diet as you have been. My ratio is 1 part boiled chicken to 4 parts 20-minute white rice. Feed only a small amount of one time, 3-4 times through the day. I would not feed any regular dog food.
2. If she is not currently taking any aspirin or nsaid products and is not vomiting, then you can give Regular Strength Pepto Bismol. The dose would be 1/2 ml/lb 2 times a day. For example, if he weighs 20 lbs, then you would give 2.0 teaspoons at each dose ( 5 mls=1 tsp). It may change the stool color to a gray-black or greenish color though so this shouldn't concern you if it happens.
Pepto bismol should not be given to vomiting dogs since it contains salicylates (the active ingredient in Aspirin) which can irritate the stomach.
3.Consider probiotics such as Forti-flora.These may be available at local pet/grain stores.
It is possible that she will need stronger anti-diarrheal drugs from your vet if the stool doesn't start to firm up within a day or so. If you have to have her seen, I would also take a stool sample to be evaluated, if possible.
I hope this helps