Kitty having Diarrhea

I've noticed that for the past week or so my 2yr old cat is having diarrhea. The food we feed her now is a brand that she's had in the past and hadn't given her problems then. The only thing that has changed is that we started giving her 1 big meal for her to nibble on throughout the day. Can that be the reason? What other reason can it be? What should I do in the meantime before taking her to the vet?

Well if it has been going on for a week then something may not be right. You should probably take her to the vet. Did you just switch back to this food? Because even if the cat has had it before, it doesn't matter, if you are switching back to a previous food from another, you still need to do it gradually or your cat could react to that change. I don't think diarrhea for a week is good though. I switched once or twice and one or two of my cats will throw up. But once.

What does this one big meal consist of? If it's canned food sitting out for 24 hours at a time, it could be growing bacteria and giving your kitty food poisoning.

I was thinking bacteria too, although it could also be worms. I'd take her to the vet just to make sure, and he can give her some proper worming meds.

As for canned food, I only give her what she can eat at one sitting and the rest goes in the fridge. I haven't given her canned food for a week because I thought that was causing her diarrhea but the diarrhea hasn't let up. What I've been doing is leaving about 1 cup of dry food for her to nibble on throughout the day. Do you think bacteria could still grow in the dry food? How would she get worms, etc? Is there a way to tell if she does have worms?

Your best bet is to run a fecal into the vet for a check. Diarrhea is a real concern because the cat can become dehydrated quickly and need subcu fluids. When my cats have diarrhea over just a few days, I take them to the vet to find out why.