Friend, when we see an increase in water intake in older cats this can be caused by many different medical conditions. It will be difficult for me to determine the exact underlying cause without a hands on examination and some testing. But, common causes are: Kidney or liver disease, diabetes, hyperthyroidism, cancer, infections, etc. to name a few causes. The vomiting is a consequence of any of the above mentioned diseases and excess water intake. Unfortunately we are very limited with what you can do at home. I really do understand how difficult it is for you. But, she really needs to be seen by a veterinarian to have blood work done to determine which of the above is causing her to drink more and vomit. In addition, supportive care (IV fluids, anti emetics, etc.). Additional treatment will be based on test and exam results. If cost is a concern look into your local ASPCA or Shelter where they often offer veterinary services at a lower cost. In the meantime this is what you can do:
1- Do not give things like Pepto Bismol or imodium. Start her on Pepcid AC (famotidine) 10mg 1/4 tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 2-3 hours. Need to give her gastrointestinal tract time to rest.
3- After 2-3 hours offer a small amount of water. Wait half hour, if no vomiting you can continue to let her drink small amounts every hour or so. Then, offer a small amount of a bland diet like rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day.
4- Follow up with a veterinarian as soon as you can for blood work and supportive care as we discussed above.
Please do not forget to rate my answer - I hope you found it to be excellent. If there’s more I can do, please use the reply tab and let me know. It’s my goal to provide you with excellent service." Thank you for your business and I hope to work with you again soon!