Metacam alone often makes cats very ill and has a black box warning now that Metacam can cause acute renal failure in cats.
I'm not a vet or anything, but I 100% agree that you need to stop using the pain meds. Would you rather your cat be sore for a few days or dead? You can keep giving her the pain meds which are probably doing absolutely nothing for her anyways because it was a spay surgery, not some sort of crazy horrible surgery... the only thing the combination of Metacam and post op NSAIDS is going to do is probably kill your cat at this point.
I'm a surgery tech and I basically refuse to give Metacam to any animals at this point because I've seen too many bad things happen from it and to be honest it does NOTHING to help the animals. Sorry, long rant, but it's a waste of money and puts animals at risk, especially kitties.
Does it say what anesthesia was used? Probably not related but I am curious as I am not familiar with the pain pills they sent you home on.
And just for the record it can take several days for the diarrhea to resolve after stopping the pain pills. The amount of time you gave for the problem to resolve was not enough.
Sorry, I'm kind of bitter about how it seems like vets really go overboard with pain meds now a days when at where I work the dogs are up and playing and happily bouncing about within 2-3 hours after their surgery. It's not some horribly blood surgery that requires weeks of pain meds like some places make it seem
unless there is some sort of special reason for it, and the vet should have made that clear at the time you picked her up. And usually the meds they go home on make them feel worse than when they left. I mean even my cat who had open heart surgery I didn't give him more than 1 day of pain meds because they made him feel like crap and as soon as it wore off he was a happy little man again.
I would still give your vet a call first thing, but if you were a client that called where I worked, this is what I would tell you to do until the veterinarian who did the surgery was contacted. It's all up to you though.