You're welcome; I hope that I can help you and Trixie today.
As I'm sure you know, there could be several different reasons why she might develop diarrhea and yet still act hungry and perfectly fine otherwise. In fact, it's an excellent sign that she is behaving normally. I agree with your vet that her loose stools are not likely to be secondary to a systemic disease because she is feeling so good but blood work might be something to consider if it continues.
In no particular order:
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.
But, I usually run a stool sample if loose stools continue for more than just a few days on the off chance that I might identify a parasite as the problem.
2. Dietary indiscretion..she ate something that is irritating her intestines as it passes through.
3.Inflammatory bowel disease although this would be a little less likely in a dog this age unless she has had intermittent gastrointestinal problems.
4. Ingestion of spoiled or rotten food. Dogs typically will vomit first and then develop looser stools later as their bodies are reacting to the insult to their systems.
5. Viral or bacterial infections. Dogs with this problem can sometimes take 5-7days before the stools return to normal. So if this is the problem, she may have a few more days to go before "normal" is seen.
...and this is just a partial list.
Since she is otherwise acting fine, then you may be able to treat her symptomatically for a few more days and she will get past this.
1. ProKolin is not a bad product to use since it does contain probiotics. But there may not be enough Kaolin or Pectin to help bind the stools. If she is not currently taking any aspirin or nsaid products and since she's not vomiting any more, then you change to Regular Strength Pepto Bismol which may work more effectively. The dose would be 1/2 ml/lb 2 times a day. For example, if she weighs 50 lbs, then you would give 1.5 tbs at each dose (15 mls=1 tbs). 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.
2 Your bland diet ratio isn't bad. I usually recommend 1 part boiled chicken (or hamburg or low fat cottage cheese) to 4 parts 20-minute boiled white rice. . Feed only a small amount of one time, 3-4 times through the day. (the active ingredient in Aspirin) which can irritate the stomach.
3. I would suggest continuing probiotics but Forti-flora may be slightly more effective since it's a veterinary product specifically formulated for dogs. It also contains more "friendly" bacteria than Pro-Kolin does.These may be available at local pet/grain stores.
4. Metamucil, at around 1-2 TEAspoons twice a day can act as a binder. Giving too much can contribute to looser stools which is why I emphasize the amount.
5. Encourage lots of fluids so she remains hydrated.
It is possible that she will need stronger anti-diarrheal drugs such as Metronidazole if her stool doesn't start to firm up within another day or so. If you have to have her rechecked, I would also take a stool sample to be evaluated, if possible.
I wouldn't be too concerned about her just yet because she's not acting sick....and this is a really important point (which I obviously keep mentioning).
But I hope this helps and eases your mind about her somewhat. Deb