Thank you for your question regarding your 7 year old Cavalier girl who seems constipated and who hasn't eaten in a couple of days now. There are a number of things that could be causing your girl's inappetance, however if she is obviously struggling to defecate and is only passing small, firm feces, then this is most likely due to the constipation as well.
Continue to keep your girl's fluids up, and you may want to make up a warm chicken
broth for her to see if this helps to encourage her to drink. For this, just boil up some chicken, remove the solids, allow the liquid to cool down, then feed her some luke-warm broth. If your girl is constantly straining to defecate, but little or nothing is coming out, then I would recommend getting her down to your local vet now if you can to have them assist. Here they can apply a 'Micralax' if needed, which can help get things moving.
The best 'at home' remedy is to pop around a half teaspoon of cooking oil on some wet food twice a day for a few days. This helps coat the poop at her terminal colon to help them slide through more easily. Lactulose syrup works perfectly for this situation, however you will need a prescription from your Vet for this.
As above, if she is really struggling still now, then your vet will be able to apply some Micralax which would definitely help in this situation. Micralax is a suppository that needs to be inserted up the bottom, and it helps to break up the poop in the terminal colon and coats it allowing the dog to poop easily within 5 - 10 minutes.
As far as longer term management of this goes for your girl, you could discuss with your vet a higher fiber diet, the addition of psyllium husks and seeds to her current diet, or the addition of another food like mashed pumpkin in to her diet.
Finally, if she starts defecating as per normal again, but still isn't eating, make sure you get her seen within the next 12 hours or so, as there would likely be something else going on here if that is the case.
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