Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Diarrhea is not a common side effect of vaccinations but can be seen this if a cat was perhaps brewing a GI bug at the time of vaccination (where the distraction of the immune system by also contending with the vaccine allowed the opportunistic infection to take root). That said, if you have potentially just changed her diet or have started offering a rich kitten food we do have to also consider that any quick diet swaps can induce diarrhea as well as rich foods are sometimes not as well tolerated by certain cats.
In regards to approaching this situation, I would first advise considering a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). These diets aim for provide nutrition to the stressed gut without making it work hard to digest the food. I would advise frequent small meals to minimize strain on the already stressed gut, and to help lower the volume of diarrhea. Usually with benign upset cases, they will settle down on this alone and we can then slowly wean them back to normal food over a week (though iif they don't settle or the loose stools recurr, then often its a hint to us that we do have an opportunistic bug that may require antibiotics to clear for us).
While offering her a light diet, do keep a close eye on her water intake and her hydration. Since she is an older lass she will not have the body reserves she used to have. This means that if the diarrhea continues, she can quickly become dehydrated. To check her hydration status to make sure she is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether she has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. ( They use a big dog but it makes it easier to see and the principles are exactly the same) If you are seeing any signs of dehydration already, then you do want to have your kitty seen by her vet before this gets out of control.
Further to this, if a cat has diarrhea just due to benign GI upset, the above is usually enough to settle it for them. But sometimes we need to use feline safe anti-diarrheals to slow things down for their gut. As I am sure you appreciate, these would not be a cure (since cures would depend on the culprit and might include antibiotics or anti-parasitics, etc.) but would slow the diarrhea to aid the body potentially absorb more water/nutrients then it would have if the diarrhea were unchecked. In regards to safe options for your lass, the one we most commonly use in cats is Kaolin (More Info/Dose) available from your local pharmacy (do avoid using any PeptoBismol or similar products with aspirin in them) or Protexin Pro-Fiber (which is available OTC at vet practices; example). Both will slow diarrhea safely for a cat and the Pro-Fiber has the bonus of providing support to the delicate good bacteria of the GI
Overall, this isn't a normal part of vaccination but can arise if there was an opportunistic infection subclincally lurking. Furthermore, if there has been a lot of diet modification in recent times, then this too can cause diarrhea. Therefore, do try her with a light diet +/- a feline friendly anti-diarrheal. Hopefully, she will settle with this but if she isn't doing so over the weekend, then its worth a word to her vet on Monday so that antibiotics can be dispensed to tackle any bacterial bugs present.
I hope this information is helpful.
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All the best,
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