Okay great, thank you for the additional info.
Runny eyes are not uncommon in cats. As she is struggling to open it, it is likely a bit uncomfortable for her at the moment.
As only one eye is affected, this raises the suspicion for a foreign body. It is always possible that she has a got a bit of dust or grit trapped inside, perhaps from her cat litter or the garden. If she tolerates it, you can gently try to flush the surface of the eye with warm saline- which may unplug anything that has lodged. This may be a two person job, an she should be rewarded with plenty of praise afterwards.
A clear rather than brown/yellow/green discharge is good, and makes a bacterial infection less likely (though not impossible). If it were to change colour, antibiotic drops will likely be needed.
Another possibility is a small ulcer. Ulcers can occur on the surface of the eye secondary to infection or trauma e.g. a scratch. The vet would be able to diagnose an ulcer by using a stain, and would need to prescribe some topical drops to help healing. The sooner the drops are applied the better, so I would certainly schedule a Monday morning appointment if possible.
It is also a possibility that Ffion has a blocked tear duct (more common in breeds such as Persians) and her tears are overflowing. Again, the vet will be able to test for this with the same stain they use to check for ulcers (if the tear duct is blocked, the dye will not run out of the nose as it should). Sometimes, the vet will flush the tear duct.
Finally, because she is sneezing, this may also be a respiratory infection. Hopefully she has a good strong immune system that will quickly clear any infection. Keep her in good form by ensuring she is drinking well and if you notice any decline in her appetite, offer strong smelling foods (she may also have a blocked nose, which can decrease a cat's appetite in some cases). If there is any sign of a blocked nose, you could also consider having her in the room with you during a steamy bath/shower to help clear things.
In summary, if the eye does not worsen and she continues to be her normal self, I would try flushing the eye once. I would then gently clean it with some warm, wet cotton wool every 12 hours or so.
If available to you (you can get one from a pet shop e.g. Pets at home) it would be worth using an elizabethan collar (buster collar) to prevent any rubbing as this will irritate the eye and slow healing.
As eyes are so important and sensitive, if things seem to be worsening at all, consider bringing her to a weekend vet.
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