So just to clarify, Katara has been completely fine up until just yesterday? And she only vomits and has diarrhea after you have friends visiting?
Has she always hid when you had friends over? Any other changes in the house? Are you feeding the same type of Science Diet? Are you near the end of a bag or have you started a new one?
I'm going to be leaving the computer soon, so will opt out and hopefully another expert can help you out.
Hi! I am a Licensed Veterinary Technician and have worked in 2 different cat hospitals for well over 10 years. I'm looking forward to assisting you! Poor Katara! Some hairballs can be surprisingly large!
The fact that she seems to feel well otherwise is a good sign though. The red/pink that you are seeing is most likely a little bit of blood, which is probably just due to the large size of the hairball and a little irritation that it may have caused when she vomited it up.
Any blood tinge color should be gone with a couple of days. If you at all think that the red specks that you saw could have resembled something other than blood, such as pieces of something she might have eaten, but should not have, then you will definitely have to monitor her closely to ensure that whatever it was passes through the other end ok.
It can be quite normal for a cat to vomit for a couple of days after bringing up a hairball. It is possible that there may be a second hairball irritating her as well though and causing her to vomit. To be safe, you can give her some cat hairball remedy paste (bought from your local pet store) to help her pass it if that is the case. The instructions will be on the tube, but you will give a small amount every other day or so until she seems to be feeling better and then you can give weekly to ensure she passes hairballs easier in the future. The paste is usually flavored to make it easier to give to your cat.
The other thing that you can do to decrease her vomiting now is to give her 1/4 of a regular Pepcid AC (Famotidine) orally every 12 hours until she seems to be feeling better.
A cat's appetite is a great sign of how they are feeling, so as long as she continues to eat well and feel well, that is a great sign! Certainly if she worsens at all or if the vomiting blood continues past a few days, then she will need to be seen by a vet. I hope this information is helpful and that she will be feeling better soon!
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Best Regards, FelineMed