There are a couple of possibilities here that may explain what is happening to you.
1. There can be problems in the eye after cataract surgery that can affect the retina in fairly short order causing relatively sudden vision changes. This includes swelling from the cataract surgery (although 18 months out from the surgery is a bit far for that to be likely) or it could be swelling in the retina from something altogether unrelated.
2. You may have a clouding of the shell that holds the lens implant. This is not an uncommon side effect of surgery that tends to show up within a year or two after the surgery. Thankfully, if it is this, there is a straightforward laser procedure that can be done which is painless and can remove that cloudy shell in a matter of minutes.
3. There may be a change in the glasses prescription in that eye which can also happen several months to a few years down the road from cataract surgery.
Both 2 and 3 above don't usually happen suddenly, but often times our recognition of their existence is sudden. Sometimes, we rub the other eye, or get something in the other eye, prompting to close the other eye and then only look out of the affected eye, allowing the patient to notice the problem all at once.
I would recommend going to see your ophthalmologist who can, with a complete dilated exam, rule out all of these scenarios and present you with the most appropriate means of treatment.
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must see an eye doctor.